I have a simple project with a drumbeat on one track, and a synth plugin loaded into a second midi track.
When playback is paused, I can play sounds using my Arturia Keystep – including with the arpeggiator and sequences saved on the device.
I can also record MIDI, as you would expect.
However, if I want to practice a keyboard part without recording, and simply press play (with no tracks armed) I can’t get any synth sounds to play. As soon as I stop playback I can play hear synth sounds again.
Doesn’t matter what synth plugin I use – same result.
In Ardour 5.x, during playback the program can play back either the data from disk (audio or MIDI) OR the incoming data. It cannot do both. So you can either listen to the recorded MIDI, or the sounds generated from the incoming MIDI, but not both.
In Ardour 6.x (not yet released and there is no date planned yet), you can do both (or either, as you choose).