I'll chime in on this as someone who up until recently was still using 8.5. Some keyboard shortcuts were so engrained in me, I didn't want to adapt to how the defaults in X3 have been reassigned, so I connected "M" to the Track Manager again, the way it was in 8.5. I know M is now for Marker, but that was F11 in 8.5, so reassigned that one also.
Only problem is while M opens the Track Manager in Track View, which is usually the only view I need it in, if I go over to the Console View which I have open on my 2nd screen, M reverts to Marker. Odd, and similar to what's being talked about on this thread.
It hasn't been an issue for me really, because I have the option checked in the Track Manager pop-up to have the TV and Console views in sync. I don't want to see different tracks in the different work spaces, so that works for me. But it is a bit hinky.
Randy B.