I also ran into this M1, M2, etc, etc, gear issue, that you can't change back from it, until you slow down enough. Why is that ? Certainly it shouldn't be a 100 % true feature.
This isn't a bug. That's a requested feature. It's there to not make you accidentally shift to D gear while driving (especially under stress). EDIT- The game should allow you to go back to D gear if your speed is below a certain limit.
But how about that system being optional, that you can trigger form the menus ? Currently it messes up the gearchanges pretty badly, since if you accidentally hit one more time and the gearbox goes into that mode, you need to ''start again''.