I've twice open the no-F5-mutes file from cold, done Melody->Mute Melody... and then added a RealTrack melody (_JAZMEDL.STY) unaltered. Both times did exactly as I would expect.

I then modified the medly just by swapping around the players: 1224 Vibes (skipped), <Silence> (chorus 1), 456 Alto Sax, 765 Guitar, 460 Trombone, <silence> x5. I left the final 458 Tenor sax for now. Said Yes to over-ride and regenerated the track. Again it did exactly as I expected.

I did a full regererate and play from the >+ button. Again it played exactly as expected.

I might add that that "exactly as expected" sounds pretty good, too, which has cheered me up no end.

For the present, at least, I'll abandon the F5 chorus mutes in favour of the menu Melody->Mute..., at least for practicing.

I would really rather like either to understand how to avoid the F5 mute failure, or if it's a real bug, helpto get it fixed.

Jazz relative beginner, starting at a much older age than was helpful.
Kawai MP6, Korg M50, Ui24R, Saffire Pro 40.
AVL:MXE Linux; Windows 11; Win8.1: Scarletts
BIAB2022 UltraPAK, Reaper, a bunch of stuff.