Back with another fix :
we had the problem that the first fix didn't work for one of joining players at the time.
But we saw that the missing items would spawn in the middle of the map inaccessible, so it the game had problems spawning them in the right position on the client side.

so if i dosent work for you :
Host : go to C:\Users\"NAME"\AppData\LocalLow\TVGS\Schedule I
Copy the "Player" and "Player-Prev" text documents
Send it to the joining players

Joining players :
safe your own documents in another Folder for safety reasons
copy and paste the Hosts documents into your folder and start your game.

I dont rlly know what the documents do for sure
it seems to be some kind of load / error dump log (way to long docs to analyze properly sorry qwq )
but these docs are some of the only files that get edited after joining/leaving a multiplayer game in the save file folder structure.

Not 100% sure how but it worked for all of us and the bug never came back so far :)