Edit: could'n make it happen when testing, so lets say it was derp from game or from me, np...
Possible bug in Big Backpack DLL?
Is it possible that your dll is 75 slots and ui is only 72 slots?
I had yesterday two cases where my stuff was missing.
I was playing single player version 5.1.2.
First one was dying with full inventory and full toolbelt, after i got all my stuff back i was missing at least my health bandages, there were in toolbelt slot 8.
Other one was when i looted stuff i didn't find them in in my inventory, those were tree seeds i was going to plant.
After i emptied my inventory to my base then i noticed those seeds again...
*I think* These can happen if dll is bigger than UI.
And container where your stuff goes when dying, then you loose some items.
You can loot with R-get all key and they go to your inventory but you can't see them until make more room and you click reorganize key.
I haven't double tested this yet but that dll size seems possible cause of what happened.
I test this if I can play tonight but others are welcome to comment to this also!