MIDI input and output device selections not immediately effective
If on Preferences>I/O I set a MIDI input device and/or a MIDI output device (and push "Apply"), these are not effective until MuseScore has been closed and restarted.
The button identified as "Toggle 'MIDI input' " affects both MIDI input and output ports.
If the "Toggle 'MIDI input' " button is pushed (being previously "normal"), both MIDI input and output ports are disabled. If it is pushed again (turned back to "normal") the MIDI input and output ports are not re-enabled. They can only be re-enabled by closing MuseScore and then starting it again. The status of "Toggle 'MIDI input' " when this is done does not affect the result.