Upgrading buildings make stuff disappear

Game mode: Singleplayer
Problem: Bug

During the 2 last sessions i have been upgrading my outpost from sanstone tier 1 into blackice tier3 and half of the placeables/ chests had disappeared when i logged back in.
So upgrading seems to be expensive!!! i lost 3 thralls (named ones) + 3 placeables and 5 chests full of stuff thanks to this issue and i’m curious to see what i will be losing next time i relogg in, since i have been further upgrading during the last session too.
What seems weird to me, is that i have been able to upgrade 1 base and a half from tier 1 into tier 2 a few days ago in the desert (about 1 week ago i think)
So i have no idea what is causing this issue and will keep on posting further feedback if i find out sthing new.

Repro steps:
1.upgrade building pieces
2.log out
3.log back in
4.boom! stuff vanished (but not everything)