I’ve had that problem and the only solution I found was to delete and reinstall.
I was running a project I had been working on for some time that had a reasonable number of audio tracks and effects, which included several tracks of chopped up drums. The first strange thing that I noticed was that the interface was a little sluggish. Strangely whenever I pressed undo it started taking a while, the screen blinked, and there was a message. I can’t remember what exactly it was, something like “project reinstated”? I then stupidly switched the iPad off and back on without shutting down Cubase first…doh! When my Pad rebooted I had exactly the same problem described by the OP.