What you see is normal. It is mounted, it's just that you cannot change it because it is part of the system security of macOS. You are running either Big Sur or Monterey, where the "Macintosh HD" drive is not accessible to the user, which is why it is greyed out. Outside Disk Utility, all will appear as if the boot drive is "Macintosh HD" and all is as it used to be, but in reality you can only write to your home folder (and subfolders) on the "Data" Volume.
Bottom line: All is well.