Duplicate of #122 but with more information and a reliable way to replicate.
Forge/Mods:
forge-1.12.2-14.23.5.2838.jar OR forge-1.12.2-14.23.5.2854.jar
MultiMine-1.12.2.jar - (Uploaded to CF on Sep 21, 2019, or "latest")
WearableBackpacks-1.12.2-3.1.4.jar

1. Start Minecraft with the above 2 mods and load into a creative world.
2. Cheat in a backpack and a chestplate. Place the backpack and wear the chestplate.
3. Hold crouch
4. Start punching/mining the backpack - Don't stop
5. Uncrouch
6. Close the game.
7. Edit multimine.cfg and set B:"wearablebackpacks:backpack-0"=true under bannedblocks* Config - https://pastebin.com/5y5PiCyw
8. Repeat steps 3-5.
9. Crash
   *I have replicated this crash without this setting however setting this from false to true seemed to cause a crash every single time as opposed to rarely.

Crash report - https://pastebin.com/XBtHg4D7

(Ideally you would have responded to the existing issue with this information, but this is alright.)

My guess is this has something to do with the backpack's break speed changing depending on whether you sneak or not (or just becoming "unbreakable" when you already are wearing one). If this is not reproducible without MultiMine, then I would think the issue is with it rather than WBs.

Sorry, in the future I'll do this.

I will try to reproduce this without Multimine more thoroughly and get back to you. As I mentioned, this seems to happen with both mods present, and the replicability of it drops severely when that setting is not set to true.

This setting is true for RLCraft, and this was the pack I used to narrow it down to MM and WB. I noticed it crashed consistently with RLCraft's configs, but not when both mods are using default configs.

Edit: I think it's safe to say this is can't be replicated without MM.

Interesting. When MM is not present the backpack is unbreakable after uncrouching (following steps 3-5). When MM IS present, however, if a crash does not happen after uncrouching then the backpack does break. The backpack remains unbreakable if crouch is held.
Edit: Seems the backpack does break with just WB, it just takes a lot longer (as if I didn't have the right tool to break it) whereas it breaks quickly in the same situation with MM installed.

I tried for a while to crash the game using the above method with just WB and had no luck. I think it's safe to say this issue is causing by MM + WB.

If you need any more information from me or wish for me to test anything for you let me know.

I'm very grateful for you testing this so thoroughly! 💚 I hope it can help if you want to report this on the Multi Mine issue tracker. Unsure if this project will be seeing updates.

That's unfortunate to hear but completely understandable. I will report this to them and hopefully they can do something on their end.

Thank you for your contribution to the modded minecraft community. :)

Hello there, also encountered this bug (player of RLCraft too) and glad the source of it was found ; It happened to me twice and I would like to report another issue (with the same bug) : when the game crashes, all the containers whose are not in the FOV of the player seems to corrupt and become invisible (you can still interact with them, they re-appear on click) and empty themselves (even after re appearing).
I didn't know where to report this... so here I am. Also I could try to reproduce it (did it twice without knowing) and screenshot the invisible stand armor/chests that it causes.
(Sorry if my english isn't correct, not a native speaker.)

I have personally seen this issue pop up as well, however I couldn't get it to happen by forcing crashes over and over again. Because of this, I can't verify this specific issue is actually causing the crash. It would seem that something else is happening, and might be another issue entirely.

It would be a lot more helpful if you could find a way to replicate this, as I have done. As you can see, the main point here is changing a config setting: B:"wearablebackpacks:backpack-0"=true. Without this config setting, the crash only happens like 5% of the time. However, with this setting it increases to 100%.

So far, I have not seen any way to get this crash to cause inventories to be deleted, nor have I been able to get inventories to be deleted using this crash. If you can pinpoint the issue, that would go a long way in getting the issue fixed. As it stands right now, it's hard to say if this crash is the actual cause of inventories being deleted, of if it's a compounding issue from another mod, or mod's config setting.