That this is a modded server with the 'Build Anywhere" mod is irrelevant, because this would happen regardless of whether I had build gates and fences on the barn, and vanilla players hide cars inside barns too.

My cars were nicely aligned before restart. After restart, one had despawned, and another was on the roof, another was propped up against my tent, and the rest were in a totally different place to where I left them.

Also a generator and construction light had despawned, leaving only the cable reel.

See screenshot.....
https://imgur.com/a/g7oO5ID