Chords track not appearing in mixer

I am new to MuseScore (3.6.2) and have scored a duet as a first project (cello, double bass). The chords are placed between the staves (i.e. above the double bass part) to provide a harmonic context for students.

In the mixer, two volume sliders, one for each of cello and double bass appear--but there is NO volume slider for the chords. In fact, the behavior appears to be that chords and double bass are on the same slider!

If I place the chords above the cello part, the behavior switches so that the chords and cello are now on the same slider (but still no 3rd volume slider for the chords track). of

So, how can I independently control the volume of each of these three elements? I seriously think I have a fundamental misunderstanding here.....or something.....handbook suggests the chord track, once entered will "appear" in the mixer?

I have included the *.mscz file for reference.

Thanks in advance for any helpful explanation or solution