It looks like I had to restart Oplab itself even after a computer reboot
Please always verify issues like this with a tiny MIDI program like SendSX
before investigating further - verifying that any
MIDI application works should always be done first before searching for a fault in a specific one (i.e. OpenMPT).
I don't know what it is but the midi plugin UX interaction seems kinda quirky. I really don't like that there is a pop-up when enabling the midi-plugin on an instrument. That should be set in the midi window settings.
What popup? The plugin GUI? It is fully intentional that this is not set in the default MIDI settings, because you can choose a different MIDI device for every plugin instance in every song. If every MIDI plugin instance would be using the same MIDI device, the plugin would entirely miss its point.