tracks that they were routed to would not. It's all around a strange issue.
edit. I've just noticed in the most recent version of flstudio (20.5.1). the issue persists to other projects until restarting flstudio completely. ...
FL instance and loading just one MIDI Out makes FL freeze.
After reboot FL works again, but once I reload that project, the random MIDI Out freeze is round the corner.
There are many MIDI Out instances in that project ...