I noticed this when a bunch of Grimoires had disappeared from Aloth's inventory when I had him rejoin me at Caed Nua. I traced my steps to see just where they disappeared, since testing it again in Caed Nua didn't remove any items.
How to re-create:
-Talk to innkeeper in Gilded Vale
-ask to see rooms
-Click the "party tab"
-Remove everyone, or just one character (doesn't seem to matter, tested it all!)
-Next time the character joins party his/her inventory is empty. Tested it with random items on every character.
I travelled to a bunch of other inns and this did not happen anywhere else, at least where I tested it. Doesn't happen in Caed Nua.
Odd note: for some reason Pallegina wasn't affected if she rejoined right back in the Black Hound.
This is early in the game, I got all companions at level 4 with very few quests done. Caed Nua is barely upgraded etc. Just mentioning in case there is some flag problems.
Will edit and upload save files if neccessary.