Describe the bug
Unlike previous tier upgrades, upgrading from Diamond to Netherite via the Smithing Table results in a brand new Netherite backpack. Everything is lost - items, upgrades, custom color, enchantments.

This does NOT occur in a test with just the mod and Forge (36.2.2 or 36.2.0 tested) in a fresh modpack profile. This was discovered to be present in the modpack Mineshafts & Monsters (MnM) as early as MnM 1.5.5_hf, and is still present in MnM 1.6.7. It's unknown why it's happening in the pack. I suspect that a mod in the pack is modifying a Smithing Table behavior.

To Reproduce
Steps to reproduce the behavior:

1. Download and launch Mineshafts & Monsters
2. Get a Diamond Backpack with items, upgrades, a custom color, and potentially enchantments, namely Soulbound.
3. Place the Diamond Backpack in the Smithing Table and add a Netherite Ingot.
4. Note that the output is a completely reset/fresh Netherite Backpack.
5. If you're daring, craft the Netherite Backpack and observe that it has no items, upgrades, custom color, or enchantments.

Expected behavior
The output in the Smithing Table results in the same Diamond Backpack's data but upgraded to Netherite tier in the MnM modpack.

Screenshots
The example Diamond Backpack and its Netherite tier output:

Versions
Minecraft version 1.16.5. Bug confirmed with mod versions 3.2.7.326 (modpack version 1.5.5_hf) and 3.2.12.337 (modpack version 1.6.7); both use Forge 36.2.2.

I am unable to recreate with just SBP, so I will need more info on what mod is causing this to happen. I suspect that some mod is overriding smithing table functionality and doesn't trigger the functionality that vanilla does causing this deletion.
But I don't have time to narrow this down to a specific mod in a huge modpack like this.

I am unable to recreate with just SBP, so I will need more info on what mod is causing this to happen. I suspect that some mod is overriding smithing table functionality and doesn't trigger the functionality that vanilla does causing this deletion. But I don't have time to narrow this down to a specific mod in a huge modpack like this.

I also have a ticket open with the modpack creator in the pack's Discord. He's going to look into it himself as well. I realized that asking you to download the pack and look into it was asking a lot, so I made sure to tell the pack creator that the bug isn't happening when it's just your mod in MC.

any update on this? anything found that requires a fix in sbp? Otherwise I am going to close this

any update on this? anything found that requires a fix in sbp? Otherwise I am going to close this

Hey, thanks for checking in (and sorry for not following up). Looks like Mineshafts & Monsters' pack creator bstylia did address this at some point, as it appears to be fixed. He did not disclose what the core cause was, but it's likely a mod or option that changed the functionality of the Smithing Table. I'm not sure what you can do to help avoid this on your end, so we can close this.