How to get rid of Macintosh HD - Data drives after fallback

You need to reformat the drive, not just "Macintosh HD" before restoring.

I have the same problem, but my drive is twice as large :-)

Hoping Catalina will resolve it when it arrives.

You can probably just delete the unwanted volume.

Catalina was working great until beta 4, where like many others on here have noted it would not finish the update. So had to do a fresh install, back to High Sierra, then to Mojave and staying there for now. When reformatting the drive, reformatted Macintosh HD and installed there but Macintosh HD - Data remained. So logged back into recovery mode then disk utility and deleted the - data volume.

Restarted and all seems to work as expected so far.