Ha!
I couldn't quite give up yet and found it has changed it's behavior after removal of various vsti's/tracks tho not the same one removed clears the issue...
I'm using a project that has:
Session Drummer 3
VSampler 32 VSTi
Rapture
True Piano
one midi track feeding external hardware and an audio track where the external audio has been recorded...
I ma also using Amplitube 3 Free on an adio track
My first removal was Amplitube...
Nothing changed
Next I removed my most likely suspect the VSampler
Nothing Changed...
Nothing changed until I removed them all, then zoom worked correctly even if I added additional audio tracks...
Next I reloaded the song and started the same process in the reverse order and after removing VSampler the behavior corrected itself...
So I will investigate further, but there is something going on here for sure...
Keni