Perhaps that has to do with the new volume structure in Catalina? This is just a guess on my part. Here is a quote from Apple's Catalina preview webpage:
Dedicated system volume.
macOS Catalina runs in its own read-only volume, so it’s separate from all other data on your Mac, and nothing can accidentally overwrite your system files. And Gatekeeper ensures that new apps you install have been checked for known security issues before you run them, so you’re always using good software.
https://www.apple.com/macos/catalina-preview/
It's in the Security section.
Macintosh HD may be the system volume, while Macintosh HD - Data may by everything else (apps, documents, user data, etc.)