It’s a ai traffic route broken. The service vehicle count will flicker 1-0 and repeat because it’s attempting to send out a vehicle, determining that destination can’t be reached so de spawns and tries again.
The flaw in the ai here is only the fact it won’t move on to the next call for service, it will instead stay forever in that loop of attempting to reach that building needing service that it can’t get to.
It’s ridiculous hard to find sometimes and no one ever wants to admit they made a boo boo but I guarantee if you have this flickering vehicle count on any of your service buildings that this is the cause and your going to have to very carefully recheck every road, 1way road etc etc. You may have to delete , remodel or move a few roads to fix it or move a building to allow vehicle access.
The service vehicle needs to leave its building, travel to the problem and then return to complete the route.
It’s frustrating but easy to avoid after you’ve had to spend any amount of time looking for the cooky lil mix ups that we all make that can cause this problem.