On a 2018 MacBook. Been going back and forth with Apple on this. According to multiple 'tree view' apps, (including one a support agent ran) I have over 550 GB of unknown ghost data.
All apps show identical numbers, within a few hundred MBs of one another, and correlate free space and folder sizes numbers shown in Finder. APFS is a mess despite plenty of time to fix the bugs. Would run HFS if the machine allowed it.. Recommend others look into this, although I've seen random ghost data before I've never seen anything like this...
I use CCC instead of TM so local snapshots aren't an issue and ghost data does not show in hidden files and folders. It's uncatalogued data that so far seems like I can't get rid of without wiping the drive... (Even then I'm skeptical.)