I met a strange problem when writing MIDI on Ardour. Some notes lose the sound ( or disappear quickly) and I don’t know why. It doesn’t work to remove them and create them again. You can watch the video to see the situation. https://youtu.be/wMeC0UKt2HI
Restarting Ardour solves this problem sometimes ( not always ). Using different instruments doesn’t affect.
It may help that one of the track is imported from MIDI, but it’s not the error one.
Same problem but only in particular projects, sometimes with specific tracks or plugins only (I have no idea if the problem is the track in that Project or the plugin)
As Paul says, there’s no fix for this if you’re using Ardour 5.12.
Your options are to either use an external sequencer, like QTractor, and use Jack Transport to sync it to Ardour or to use one of the nightly builds of 6.0-pre and see if it works better.
6.0 isn’t even in the beta phase yet so there may be bugs lurking around but I’ve used it for testing purposes and it seems to work pretty OK.
I haven’t used any MIDI features though, so YMMV.