Hello! I'm having this issue in a modpack, specifically Monifactory, which is in game version 1.20.1.
Putting a mending item in an EnderIO Fluid Tank with XP Juice in it will cause the XP to only somewhat fix the mending item, and then it gets put in the tank's output slot. Putting another item into the tank without removing the first seems to instantly void the prior item in the output slot. Picking up the tank and checking its NBT data with data get entity @s SelectedItem, the tools seem fully gone. The log provides essentially no information from what I can see, but I'm submitting it since I don't know anything about making mods and I don't want to get yelled at. Line 2226 is where I start spawning several golden axes with which to recreate the bug, which I was successful in replicating as described above, though these axes were able to fix themselves to full durability.

pls tell us version you by following down below Other mods just modpack version in there

Of course, sorry! I'll add the log to the bottom which contains everything, but I'll format an additional section below in the same style you've provided an example of.

• Operating System: OS Windows 11 arch amd64 version 10.0
• Java Version: java version 17.0.8 by Microsoft
• Minecraft Version: Forge mod loading, version 47.3.7, for MC 1.20.1 with MCP 20230612.114412
• Version: enderio 6.2.7-beta
• Other Mods: 224 others for 225 total, but if you search Loading 225 mods: in the log you can see them all.

latest.log