Most vehicle repairs seem to work normally, but about 5% of them cancel before reaching the completed %. (Eg. the game says the repair will take the vehicle from 60% to 15%, but then it stops repairing at 35%)

This is with vanilla repair stations. I have two and this happens with both of them. In both cases the vehicles are parked in an attached vehicle depot.
"Allow the repair cancel when long idle" is not ticked.
Other vehicles seem to continue repairing normally at the same time.
I tried leaving the vehicle in the repair station for a year so it could be repaired again, and the same thing happened.