I just stumbled onto this thread searching for fixes for a separate VCA issue.
I wrote a thread about a big very similar to this recently. Stopping the playhead over VCA automation at -inf and making certain edits in the project window (alt-drag copy pastes) or routing changes in the mixer will sometimes cause this exact issue. It cannot be ctrl-z’d and your mix balance is. The only option is to revert to previous save. God help you if your last save happened to be in one of those no-go-zones.
It is a DEVASTATING bug that has cost me hours and hours of wasted work. In my opinion, it is WAY too easy to bork your entire mix in Cubase. I’ve done it with this bug, I’ve done it using the Time Warp tool, I’ve done it confusing Musical Mode and Musical Timebase, I’ve done it using audio warp quantize on multi-mic drums not knowing the potential for phase decoherence, I’ve done it by mis-clicking the “Activate/Deactivate All Read [Write] States” buttons in the project window. I’ve actually developed a fair bit of anxiety having to use any of the more advanced features as a result, constantly creating save iterations as a result.