I noticed it on Sakhal when i found one of these vehicles: Olga, Ada, Gunter, Dirtbike, boats.

When you get in, you can see that the tank and the radiator are completly empty. Common denominator in the cases above seems to be that they all allready have been used, because i found them in the woods or crashed on the road. Only once i found a boat with half a tank, i'm guessing it just spawned.

The only thing i can think of which causes this issue is, tanks get emptied when people are being too far away from the vehicle or after the restart.

I tested it with a boat with a half full gas tank. I went to a remote island where i parked the boat hidden away between ice sheets (tank was still half full). Came back the next day (roughly 20 hours later). The boat was at the exact same place where i left it, but the fuel tank was on red.

Same with the gunter. Filled the gas tank to a quarter and the radiator fully, then i stashed it in the woods. Came back later to use it and everything was empty. This also happened with the dirt bike.

To reproduce maybe try getting away a certain distance from the vehicle then coming back or checking back after the restart.

Fuel tanks and radiators not empying themselves.