This happens with “Lock/Unlock Track” key-command only and not by clicking the lock button.
Tested with Group and FX Tracks.
Reproduce:
Create empty project
Add a mono or stereo Group channel
Change automation line from Volume to Mute
Change back to Volume
Hit command
Result:
The “W” icon (write) is deactivated but the Lock/Unlock button does not light up and sometimes Cubase crushes.