Hello Globox1997
I found a compatibility issue with Inventory Backpack https://www.curseforge.com/minecraft/mc-mods/inventory-backpack
It makes so that the pants and boots slots cover some slots of the backpack inventory, making them inacessible.

Is there a possibility for you to fix this issue?
(maybe unrelated, but the mod Trinket of Undying (Fabric) https://www.curseforge.com/minecraft/mc-mods/trinket-of-undying-fabric also adds a custom slot but i never found UI issues with that mod. Most likely it's the way it's implemented, so maybe have a look at the source code? I really hope i'm not coming out as mean or anything, i'm just trying to present what i think it's a possible improvement to your mod)

Does manually entering an offset for the slots help with this issue?

Try as @ScoobyVI mentioned. Otherwise I have no plans to add compatibility to Inventory Backpack. Ask their dev for compat.