Not really a bug, but in my opinion the current implementation could be designed better;

Imagine you are hosting an event, and you don't want to password your server, instead you want to give visitors possibility to stay as spectators. What happens after the winner crosses finish line, there is immediately spectators joining the track which is not good at all - there are still plenty of cars running their last lap, some of them may run out of fuel or out of tyre wear. Its wrong to enable the joining in that moment. Imagine if your event is running on open layout where you can't put route checkers over the grid, anyone who don't care, can run the wrong direction right from the pits.

So what I am asking, if I configure midrace joining disabled, it should stay disabled until everyone has finished his race. Has this any sense?

Yes, sounds like a good idea