0. ALWAYS PROVIDE INFORMATION ABOUT MODLOADER (Fabric / Forge) AND MOD VERSION (For Example 1.19.2-8.2.10)
   Using NeoForge 21.5.75 for Minecraft 1.21.5
   Mod version: 10.5.1 (latest as of this post)

1. Describe the bug
   Noticing unusual behaviour when interacting with a tankless backpack that has no tank upgrades. It seems normal before interacting with it on the ground and suddenly (presumably after opening inventory) the tanks appear.
   The inventory also becomes glitched, see screenshot, attempting to render empty tank slots.
   This does also cause the backpack's hitbox to change as if the tanks are actually there, when in fact they are not.

2. Write steps to reproduce the bug
   a. Place your tankless backpack on the ground, any will work.
   b. Attempt to interact with the backpack to open its inventory. Notice how the backpack appears normal here.
   c. Close the inventory afterwards, doesn't matter if you interact with the backpack here.
   d. Notice immediately that the tanks have "appeared", opening the inventory now will show the greyed out fluid tanks there as well. Continues to persist till backpack sleeping bag is deployed/folded or backpack is broken.

3. What is expected behaviour
   Assumed it would open inventory as normal and would still appear the same both before and after opening the backpack.

4. Screenshots (If applicable)
   No tank upgrades installed, yet backpack seems to act as if they were there... sort of?
   

Can't reproduce this using provided setup, do you use additional mods

Well... I am rather stumped on this, but have noticed something rather peculiar.
It only occurs while having a backpack equipped, and said backpack has a fluid tank upgrade installed while interacting with another backpack placed on the ground. This was only consistent in the world I discovered it in. I have made a fresh world now and am no longer able to replicate this. I'm suspecting the world I had this occur in might have been corrupted.
I have a feeling it may just be the result of mod changes to said world.

I do; this is currently part of my custom modpack setup, though I'm not sure it would be causing the issue.
Will try running tests on my end to identify the troublesome mod(s) and let you know what I find.

The full mods list of my modpack setup is the following:

• 3d-Skin-Layers [1.7.5] by tr7zw
• AI-Improvements [0.5.3] by BuiltBrokenModding
• AppleSkin [3.0.7+mc1.21.5] by squeek
• Architectury [16.1.4] by shedaniel
• Balm [21.5.17] by BlayTheNinth
• Bedspreads [9.0.0+1.21.5] by Illusive Soulworks
• Better Tridents [21.5.0] by Fuzs
• Block Runner [21.5.0] by Fuzs
• Catalogue [1.12.1] by MrCrayfish
• Chat Heads [0.13.18] by dzwdz, Fourmisain
• Cherished Worlds [12.0.0+1.21.5] by Illusive Soulworks
• ChoiceTheorem's Overhauled Village [${version}] by ChoiceTheorem
• Cloth Config v17 API [18.0.145] by shedaniel
• Clumps [23.0.0.1] by Jared
• Comforts [11.0.0+1.21.5] by Illusive Soulworks
• Completionist's Index [21.5.0] by Fuzs
• Configured [2.6.1] by MrCrayfish
• Controlling [23.0.2] by Jaredlll08
• Cooking for Blockheads [21.5.6] by BlayTheNinth
• Corpse [1.21.5-1.1.7] by Max Henkel
• Crafting Tweaks [21.5.1] by BlayTheNinth
• Cupboard mod [2.9] by Someaddon
• Curios API [11.0.1+1.21.5] by C4
• Cut Through [21.5.0] by Fuzs
• Distant Horizons [2.3.2-b]
• Dungeons and Taverns [1-v4.7.2] by NovaWostra, Konci, MiziraGamez, Whity
• Dungeons and Taverns Ancient City Overhaul [1-v3.1] by NovaWostra, Konci
• Dungeons and Taverns Desert Temple Overhaul [1-v2] by NovaWostra, Konci
• Dungeons and Taverns Jungle Temple Overhaul [1-v2] by NovaWostra, Konci
• Dungeons and Taverns Nether Fortress Overhaul [1-v3] by NovaWostra, Konci, MiziraGamez, Whity
• Dungeons and Taverns Ocean Monument Overhaul [1-v2] by NovaWostra, Konci, MiziraGamez
• Dungeons and Taverns Pillager Outpost Overhaul [1-v3.2.1] by NovaWostra, Konci, MiziraGamez
• Dungeons and Taverns Stronghold Overhaul [1-v2.3.1] by NovaWostra, Konci
• Dungeons and Taverns Swamp Hut Overhaul [1-v2.2] by NovaWostra, Konci
• Easy Anvils [21.5.1] by Fuzs
• Easy Magic [21.5.0] by Fuzs
• Easy Shulker Boxes [21.5.0] by Fuzs
• Echo Chest [21.5.1] by Fuzs
• Enchanting Infuser [21.5.0] by Fuzs
• End Remastered [6.1.3] by Jack Bagel
• Entity Model Features [2.4.4] by Traben
• Entity Sound Features [0.6] by Traben
• Entity Texture Features [6.2.13] by Traben
• EntityCulling [1.7.4] by tr7zw
• Essential Mod [6661558]
• FallingLeaves [3.0.0] by Cheaterpaul
• Farming for Blockheads [21.5.1] by BlayTheNinth
• FastLeafDecay [35] by Olafski
• Ferrite Core [8.0.0] by malte0811
• FirstPerson [2.4.9] by tr7zw
• Framework [0.11.5] by MrCrayfish
• Friends&Foes [4.0.6] by Faboslav
• Friends&Foes - Beekeeper Hut [3.0.0] by Faboslav
• Friends&Foes - Flowery Mooblooms [3.0.0] by Faboslav
• Geophilic [3.4.2] by bebebea_loste
• GlitchCore [2.5.0.1] by Adubbz
• Goblin Traders [1.11.1] by MrCrayfish
• Hopper Gadgetry [21.5.1] by Fuzs
• Illager Invasion [21.5.3] by Dricky, Safro, Zeckemith, Fuzs
• Inventory Essentials [21.5.1] by BlayTheNinth
• Iris [1.8.11+mc1.21.5] by coderbot, IMS212
• Jade [18.2.0+neoforge] by Snownee
• JamLib [1.3.5+1.21.5] by Jamalam
• Just Enough Items [21.3.2.22] by mezz
• Kobolds [3.2.5] by Jusey1
• Lithium [0.16.3+mc1.21.5] by 2No2Name, JellySquid
• Lithostitched [1.4.8] by Apollo
• Lootintegrations mod [4.7] by Someaddon
• M.R.U [1.0.17+1.21.5+neoforge] by IMB11 (mineblock11)
• Mighty Mail [1.2.1] by MrCrayfish
• Mouse Tweaks [2.28] by Ivan Molodetskikh (YaLTeR)
• MrCrayfish's Furniture Mod: Refurbished [1.0.16] by MrCrayfish
• Nether Chested [21.5.0] by Fuzs
• NetherPortalFix [21.5.1] by BlayTheNinth
• Noisium [2.6.0+mc1.21.5] by Steveplays28
• NotEnoughAnimations [1.9.3] by tr7zw
• Overflowing Bars [21.5.0] by Fuzs
• Paper Doll [21.5.0] by Fuzs
• Particular [1.1.4]
• Ping Wheel [1.10.2] by Luken, RXJpaw
• Puzzles Lib [21.5.12] by Fuzs
• Reese's Sodium Options [1.8.3+mc1.21.4] by FlashyReese
• Resourceful Lib [3.5.0] by ThatGravyBoat, Epic_Oreo
• Right Click Harvest [4.5.3+1.21.5] by Jamalam
• Searchables [1.0.2] by Jaredlll08
• SeasonHUD [1.13.3] by IanAnderson
• Serene Seasons [10.5.0.1] by Adubbz, Forstride
• Sheep Variety [21.5.0] by Fuzs, JOHNM
• Simple Voice Chat [1.21.5-2.5.30] by Max Henkel
• Skeleton AI Fix [21.5.0] by Fuzs
• Sodium [0.6.13+mc1.21.5] by JellySquid (jellysquid3), IMS212
• Sodium Dynamic Lights [1.0.10] by toni, LambdAurora
• Sodium Extras [1.0.8] by toni
• Sodium Options API [1.0.11] by toni
• Sound Physics Remastered [1.21.5-1.4.12] by Sonic Ether, vlad2305m, Max Henkel
• Sounds [2.4.11+1.21.5+neoforge] by IMB11
• Spawn Animations [1.10.1+mod] by Tschipcraft
• Spyglass Improvements [1.5.8] by juancarloscp52 (Im_JC)
• Stoneworks [21.5.0] by Fuzs
• Storage Delight [25.03.26-1.21.5-neoforge] by Axperty
• Straw Statues [21.5.0] by Fuzs
• Stylish Effects [21.5.0] by Fuzs
• Sword Blocking Mechanics [21.5.0] by Fuzs
• The Lost Castle [2.0.0] by Jack Bagel
• Tiny Skeletons [21.5.1] by Fuzs
• Traveler's Backpack [10.5.1] by Tiviacz1337
• True Ending: Ender Dragon Overhaul [1-v1.1.4b] by limesplatus
• Visual Workbench [21.5.0] by Fuzs
• Waystones [21.5.7] by BlayTheNinth
• Wooden Shears [1.21.4-3.4.0.1] by Cech12
• YDM's Weapon Master [4.2.6] by YourDailyModder
• YetAnotherConfigLib [3.7.1+1.21.5-neoforge] by isXander
• lootintegrations_dnt mod [1] by Someaddon
• underground_village [1.5.4] by Reggarf

So it's unable to be reproduced on another world?

It appears that way, yeah, both with taking mods out to troubleshoot it and making new worlds to test.
I'll close this one for now, will reopen if it comes back during playing.