So I think I know what's going on here, but wanted to report it as there are some counterintuitive interactions.
I originally (many years ago) created my MC library on Windows and later migrated to Linux. I have several albums in my collection that begin with an ellipsis (...), such as Metallica's "...And Justice For All". I used MC to place all of my albums in folders with their correct album name, but I just recently noticed that none of the albums with a leading ellipsis were in my MC library anymore. I confirmed that they were on disk, but they weren't in the database and MC would not import them. Renaming the directories to remove the leading ellipsis results in all albums importing normally. I tried creating a new folder with a leading ellipsis, and nothing imported. Files with a leading ellipsis seem to import normally, only folders are affected.
I suspect this is a Linux specific issue and maybe MC is ignoring folders with a leading dot on Linux because that's how *nix systems identify hidden folders, but that has an unintended consequence of sweeping in albums that are in folders that have any number of leading dots. I note also that the rename, move, copy tool will happily rename the folders to have a leading ellipsis, which doesn't seem to cause the albums to be removed on the next auto-import run, but presumably would prevent them from importing if one needed to re-run the imports from scratch (which is what I'm guessing may have happened to me?).
If MC is ignoring "hidden" folders, I'm not sure that's necessarily a good practice (how commonly do media folders even have hidden folders that folks don't want indexed?). I think it's also meaningful that it doesn't also ignore hidden files (i.e. the behavior is inconsistent). I'd prefer that MC just index what I point it at with no reservations, but if ignoring hidden folders is desirable behavior I'd suggest that rename, move, copy should avoid writing file and folder names with leading dots, the same way that it avoids including symbols that will create issues for MC or other programs (i.e. replacing certain brackets with underscores, etc.). That way folks could run an RMC pass and be confident that the files will all be visible.
In any case, just thought I'd get it out there because it was unexpected behavior and I'd been inadvertently missing six or seven albums for who knows how long :-)