I can see this has been reported on XBOX and PS as well so it seems widespread. I have done some testing in staging and fuel is only preserved if the vehicle is rendered in by player on restart. All vehicles are spawning with 0 fuel and engine is not respecting commands to insert fuel either.

started vanilla server as well as modded staging branches, added scripts to spawn all vehicles at full then check levels, all stayed at 0. Only success was a truck rendered in during a test, fuel persisted through restart. Could also be affecting coolant levels. After logging out and leaving vehicle unrendered, the restart wiped all fuel from covered truck. I suspect the same would have happened to coolant if truck had a radiator.

Tested on vanilla and modded staging server with various settings.