I just installed Ardour 5 and loaded an Ardour 4 Session.
It seems that (only for the MIDI tracks) the tempo has doubled, i.e. the MIDI plays at double speed. If I change the meter from 6/8 to 6/4 all is well for the MIDI.
But obviously for my songs with meter changes this would be a lot of work, since I would have to halve the bar positions of the different parts.
In some configurations, tempo in ardour can be expressed as beats per note type.
It is possible that your original file expressed tempo in eighth notes but this has somehow switched to quarter notes.
Could you check this please?
Thanks for taking this up (and sorry for the delayed feedback). Does that also mean it should be in 5.3? I just installed it, but still have the same issue. (I’m on OS X, if that matters).