I don’t have the More (only the OPN 1), but the fact is that the More does not give you a hard option to mute (using one of the hard buttons on the aid), so it can’t be a hardware issue, or at least not a hardware button related issue.
It may be a bug in the software, but then it must be a random bug and not a consistent bug that can be reproducible to allow an easy fix. Most likely, also based on @cvkemp 's input that he has the opposite unmuting issue, that it’s a random interference issue due to some stray signal, especially if you notice that it only occurs in a few specific places like only at the gym or in your car.
I’m not sure what can be done by Oticon to deal with random interference, except to maybe update the communication link to make the handshake more robust from interference. This would probably require not just an update to the ON app, but a firmware update to the More as well.