No idea. It is random and it happened not the first time since 1.19 (last wipe).

I had two of the same backpacks (alice) in my hands. Upon relog, the one on my back disappeared, I had only one in my hands.
It's not the first time it happens. I noticed also if the backpacks are filled with the same type of riffle AND all of them have flashlights attached then sometimes it disappears upon relog.

The flashlights are dropped on the floor (all of them) and then the backpacks disappears... It's random. There was no server restart, no force to close the game (ALT+F4) etc. Just regularly switch off the game and wait for time out. I'm attaching a screen of what I had in my inventory when logged out last time (yesterday?).

Yes it's edited in 'paint' this is how I update my stash each time I add something (yes, I'm a hoarder heh.. I stash stuff, no it's not duped, I'm just a perfectionist and a taxi driver, I have a lot of spare time to hoard instead of PvP. I'm happy to give my steam credentials so you can check my account for activity, etc, just saying)

https://photos.app.goo.gl/kK1912HrEXudTCrY7 before

https://photos.app.goo.gl/pU8nYSerSKyjm6dv9 after

Just logged in to the game