If I freeze a simple instrument track (VI), I can still see the MIDI in the Staff View.  Editing it won't change the recorded/frozen part, but it is helpful to see it.
 
If I have separate MIDI and audio tracks (for example, two MIDI tracks and one audio track for RealGuitar), when I freeze the track, the MIDI tracks disappear.
 
I actually don't remember this happening before.  Has it always been this way?
 
It is inconvenient when I want to look at the guitar chords while arranging strings or some other instruments.  Similarly, I may want to see the drum MIDI when working on a bass part.
 
Since I lose audio when there are too many unfrozen VIs, this means I have to keep freezing and unfreezing tracks.
 
Thanks.

Are you sure that the 'Delete Associated Tracks' option isn't selected on the freeze track window ?

Thanks.  The HIDE MIDI TRACK option was selected, although I have no idea how since I don't remember ever opening that options window.
 
I think this solves it.  Much appreciated.

Pleased to help :-)
 

That is the default status of the command (hide), so it likely got reset to default at some point?