Alright so I tested this on a private, passworded server so no one else was joining and here are my results. I found a bus and sedan near Cherno, fix them up slightly and log off. When I get on the next day both vehicles were gone so I headed up north where I found a offroad. I fix it up but am unable to get it to move so I restart the server. It is still there and I eventually get it moving and head to NWAF. I park at a helicrash to gather loot and leave it parked for around 5 minutes, I then restart the server. When I re-enter the server I find the offroad is gone. It seems vehicles on the server completely reset at times, this is a big issue and I hope for a hotfix of some sort.
Edit: I have done a lot of testing and I have confirmed even if you do not restart your server, all the vehicles can completely reset still! I turned off automatic restarts and found them still resetting.