stopping playback plays a huge MIDI cluster
Stopping playback sometimes issues an enormous cluster of MIDI notes to output. (Needless to say this is unbelievably loud and disturbing.) It sounds like MS is trying to send 128 note OFF messages in a loop, but instead it is sending 128 Note ON messages. The notes can be heard sweeping across the whole MIDI range in a fast loop. The phenomenon is only audible when MIDI output is set to an external destination like the IAC bus to be received by an external synth.
OS: macOS High Sierra (10.13), Arch.: x86_64, MuseScore version (64-bit): 184.108.40.20671, revision: d2d863f