In case this helps someone else… one of my synths doesn’t get a note off while its arpeggiator is running unless the region containing the MIDI notes controlling it ends after the final note. So if you have a synth not getting a note off, try moving the region end past the end of the last note. (So that there’s blank space in the MIDI region after the last note.)
What version? This should not be necessary - region end triggers “resolution” of all notes currently on from within that region …
I’m using Ardour version 8.10.
I’ve run into that same issue intermittently (albeit rarely) myself, over several versions, including a couple days ago. To keep my main grid (…playlist? the one you interact with after pressing G) tidy I turn the grid off for a moment in Edit Mode and shrink the last note in the pattern slightly to achieve the same effect. This could potentially interfere with legato I think, but hasn’t been as issue for me.