Every time I open my session, my MIDI tracks / ACE Fluid Synth break

I’ve got a session with one audio track and several midi tracks. I have removed all automation and MIDI bank events from the MIDI tracks and when I add ACE Fluid Synth and a soundfont file, I can get these MIDI tracks to produce MIDI sound.

Then I close ardour and reopen the session again.

Now the MIDI tracks are silent. The signal meter shows signal, but the L/R audio outputs are completely flat. I can remove ACE Fluid Synth from each, add it back again + the soundfont and (voila!) the MIDI audio is back.

Then I close ardour and reopen the session again.

Now the MIDI tracks are silent again.

What is happening here?

(edit: FWIW, the MIDI tracks with reasonable synth (not ACE fluid synth) are fine and always audible.)

(edit 2: The soundfont file and all the settings inside all the ACE Fluid synth instances appear normal after closing and reopening ardour.)

(edit 3, unrelated: ctrl+up/down do not move selected tracks up/down, at least not on Ubuntu 22/Gnome fresh git pull/build of Ardour 4.5.x)

(edit 4: If I create a totally fresh MIDI track (not imported from a MIDI file), then create a MIDI region and copy-paste all the notes from one of the “broken” tracks, add ACE Fluid Synth+soundfont to it, restart Ardour, that NEW track is fine and seems to always be, no matter how many restarts. The MIDI tracks imported from a MIDI file remain problematic. Stated another way, creating a fresh MIDI track with the old notes pasted in somehow eliminates whatever is causing the sound to drop on Ardour restart in the original MIDI track.)