All notes off overkill?
When MuseScore commences play, or after play is ended, it sends, on every MIDI channel used in the score, a MIDI All Notes Off message. The need for this before play is questionable - it would seem to only take care of the case in which, when play commences, the synthesizer is already sounding a "stuck" note (what an organist would call a "cipher") - not a likely (or tolerable) situation. But perhaps it is good to be scrupulous - "you can't be too careful."
But, as there is apparently insufficient faith in the power of the MIDI All Notes Off command, MuseScore, before play commences, and after play, also sends MIDI Note OFF messages for all 128 possible MIDI "notes" (pitches), over all MIDI channels in use for the score - a potential prefatory utterance of 2048 Note OFF messages.
At the very least, this result in a sometimes mystifying delay of couple of seconds before we really hear any of the notes of the score.
But that is only part of the "problem". "We" often have to employ, for several reasons (including the forensic testing so beloved to this office) a "MIDI loopback driver" (e.g., MIDI Yoke, LoopBe) that allows two MIDI-capable applications to interconnect (an otherwise impermissible same-gender coupling). If this setup is done improvidently, the result can be a "feedback loop" around which a MIDI message or a group of same can mindlessly course at the highest rate accommodated by the players.
To guard against this, many such drivers incorporate a "feedback loop circuit breaker" feature. This is often implemented in a simple way: if the flow of MIDI messages is at an average rate above some threshold for a certain period of time, the driver "mutes" the path to interrupt the phenomenon. The user is given some indication of this (often too subtle) and has to reset the mute (hopefully after correcting the improvident configuration) before the driver will work again.
Some "loopback" drivers allow us to disable this "feedback loop circuit breaker" functionality if it is not needed (and is troublesome), but others don't.
The flood of MIDI Note OFF messages that comes at the start of play (or at the end of play), especially when multiple MIDI channels are involved) can well trip the "circuit breaker" of a loopback driver.
I suggest that the sending of MIDI Note OFF messages for all 128 MIDI "notes" at the beginning and end of play be re-examined.