I see your post is from over a year ago.. well.. 2 years and last comment is over 1 year.... I AM HAVING THIS SAME PROBLEM TODAY!!! MARCH 27, 2021...and this....
It disappears while we are still in the game... I mean like we went to the next town over to look for loot and we were gone from base maybe an hour and almost everything was gone. A few walls and 2 out of 7 doors where there.... vehicle despawned as well. It had been if our server restarted but never in game... even the water pump we put in is gone... I mean WTF!? And by no means was there a 45 day period...