Why tracks are missing from the mixer section? (in Tracks/All modes)

As seen in the screenshot, some tracks do not appear in the Mixer section (opened by pressing 'x'), and it doesn't matter which mode the mixer is on - Tracks/All - some tracks just do not appear there - they are not hidden.

I can view them in the mixer section only in Single mode when they are selected in the timeline, but that's not very convenient.

However, when opening the mixer window (pressing ⌘+2), I can see these missing tracks, but because it's floating on top of the main window it's not great to work with it on a single screen.

What's the difference between the two mixer views? and how can I fix it?

Thanks!