Usually on FiveM (tho I am unsure with the framework you run on), any upgrades to a vehicle means you have to store it once before taking it out again, so the database can save the changes, the vehicles doesn't match what it is saved as thus you can't lock it
Confirmed fixed and working in the development build. If this happens again let me know, and I'll relook at it when I come to rewrite the vehicle modification system.