This is a weird edge case, not a priority bug.
I made some mistakes when I was setting up my directories: I exported my library from Sooloos but some of the stuff was not well organized so I wanted to copy in my original rips or downloads, and I didn’t realize that file names are different, so I got a lot of albums with duplicated songs, two files with different file names for the same song. No automatic way of doing large-scale cleanup. Idiot move. So I had to do a lot of manual cleanup, a bit here and a bit there as I pull together the energy.
Ok, so I delete one set of the duplicates from a few albums. Usually works out ok, but on some Keith Jarrett albums, it seems as if I delete the track version that Roon had decided was it, and I left the one that Roon had chosen to ignore. So the result was that the albums showed up, but they were empty, no tracks in them. And no way to rescan. I copied those folders (which did contain a correct set of files, remember) to another watched directory, and everything was ok, the empty album disappeared and the correct one appeared.
Not an important bug, not too many people will repeat exactly my screwup. But it might be a useful indicator of a mistake in the code: the result was a nonsensical state that should not exist, an album being empty.