Repair/Fuel status of Novistrana spawned vehicles resets after every restart.
I thought I was going crazy, after fixing up the MG Bus in Alexander twice, only to find it back at the spawn spot all busted up again.
I also fixed up the Beemer across the bridge north of Alexander. Same.

So to make sure, 5 mins before restart I fixed up the red VW Golf at Vladimir, put 40L of fuel in too. And moved it to behind the hospital. Waited for restart. Logged back in immediately.
And the vehicle has reset to it's default state and position.

Maybe this particular server has some unusual combination of admin settings that is causing it ?

Sounds like a bug to me. On my server the Land Rover with the 50 cal. on the back of it has gone from Novistrana and hasnt spawned back since so it seems like it is your server only. I will check on my server tonight, build up a vehicle and see what happens.

Are you sure that nobody is destroying your vehicles in your absence

That's what I thought. Even though I knew that no other players had been on for maybe 10 days. That's why I logged back in immediately after restart just so I knew for sure.

Does a restart apply 'time damage' to a vehicle parts, in the same way Epoch can be set up to apply damage to building parts at restart ? If so, because the tyres I am putting on these vehicles are super low health, maybe the restart damages them to the state where they are burst and the server considers the vehicle wrecked, and respawns it ? But then, if that were so, it hasnt affected any of my mainland vehicles, only the Novistrana ones.

I do not believe there are any restart 'time damage' but I havent played overpoch so I havent seen it before. I for one have not used my Rodina fuel truck for ages now but it hasnt lost any parts after restarts.

I think its a bug on your server.

But doesn't every Origins server use the same files ? I don't know, I just thought that the Devs always tried to keep Origins 'pure' and true to their vision, so no modding, and all servers are the same (bar Origins Guard admin ON/OFFs) ?

I believe all servers are the same unless specified mods are added by the server owner but as these are all hosted by Lag Free I dont think there are any differences.