BIAB 2021, Version 809, 64 Bit, Windows 10
If I am using a midi style with a track using the harmony feature, it is unpossible to mute this track. It shows "muted" but it plays further on. If I change the track to "no harmony" this effect is gone.
I reported this bug already to PGMusic.

Maybe my post helps some other users, because it took me a while to find out that this is conected to the harmony feature.