I might be wrong based on standard conventions for writing out shortcuts, but I think that the tooltip text for markdown/rich text toggle is unclear.
It includes a plus sign (+) between the two keys that need to be pressed whereas the other shortcuts do not have a plus sign.
Without + for bolding:
With + for the markdown/rich text mode toggle:
I couldn’t get the toggle to work at first because of this.
I realized that the shortcut for the toggle is ctrl (at least on Mac) and M at the same time with no + in there at all.