Soloing a MIDI track no longer works

Getting Stuff Done How do I .... ?
#1

Hi all,
a strange thing happens: in the current multitrack project, when I set “Solo” in a MIDI track (and only that one), other tracks play instead. What settings to change?

[Ardour 8.1 on Linux]
Thank you,
a.

#2

I got it: some tracks had “Iso” set. (I never set it…)

iso

I unchecked it and solved the problem.
a.