A screencast is worth several words: https://imgur.com/j2iaXkX
This issue has been referenced before in this forum, thread “Ardour cuts MIDI note on region start”, and it was surmised that this is fixed in Ardour6. Well, this is Ardour-6.0.0.
Note that this doesn’t occur with all regions, e.g. the region just above it also has a MIDI note on its start, and behaves correctly.