I erased the RealTracks to stop them confusing things.
I went through the F5 bar settings, and set them again. Still no mute.
I tried setting "All Instruments" "back to normal", though that seemed to do nothing.
I set melody on every chorus to Mute. No mute.
I set melody every choris to "Back to normal" then to Mute. No mute.
I did Reset ALL Bars and set Mute again on melody on Chorus 2 and Back to Normal on Chorus 6. No mute.
It appears that melody mute now does not work on this song.

Prior to doing the Reset ALL Bars, I'd also been aware that there was a star against chorus 1 bar 1, which I think means it believes there's a change at that point, but I could see no change and hadn't knowingly(!) put any change in it. The reset ALL cleared it.

It's a struggle :-/

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.