Is there necessary create another MIDI track with the same settings
No. Just tested with a blank project and adding a single midi track and the same issue occurs.
I’d it necessary to wait? What happens if you disable and re-enable the MIDI track immediately?
Yes it only seems to happen after the midi track has been disabled for a few minutes (approx 5 minutes, sometimes longer). If I disable and re-enable the MIDI track immediately then the midi send output routing is preserved correctly.
Could you try in Safe Start Mode?
Have just tried in Safe Start Mode with preferences disabled and am getting exactly the same issue.
Incidentally, the problem only seems to apply to midi send output routing - the main output routing of the midi track itself is preserved correctly upon re-enabling.