if you have full USE inventory (no free slots), and a stack of for example 90 elixirs. another character drops a stack of 75 elixirs, press loot key, you pick up 10. The rest are gone. i only tested USE inv, it might happen with ETC stackables too.
This bug is verified by Sila, and it was added that the bug only affects consumable, not things like scrolls and stuffs. Thanks for your report.
I believe this was fixed a lil while ago: https://royals.ms/forum/threads/new-source-update-28-11-06-2017.93062/