This was a result of a change in the way the tag finders work, which in turn exposed a long-standing bug in the multifile tagger (i.e. it does not check whether the IMSLP number given is valid). Incidentally, the tag finders should be much faster than before (before as in before the 1.15 upgrade).
The bug in the multifile tagger has been fixed, but the errors from the bug still remain. I thought the 6 digit tag was the only error, and so I removed it manually. Apparently it is not. But that is OK, because the system will overwrite the current tag when someone uploads IMSLP #37278.