I'm reporting a bug? in 2.91 which disappeared when I downloaded 2.91 a second time and reinstalled it. So it may not be a bug, but I thought you would want to know about it, since sometimes issues are related to uninitialized RAM or some transient boundary condition. Both downloads (mp3tagv291setup.exe) were 3,531KB, with the same MD5 checksum, so the install files weren't at fault. Possibly the installation process glitched?
The problem was that when auto-renumbering a set of 15-20 folders, with "restart numbering for each folder" selected, numbering was restarted several times within each folder. The restarting appeared to be inconsistently related to the existence of numerical digits in the filename, but that's just a wild guess. In any case, the problem was consistent until I reinstalled, and then restarting numbering for each folder worked correctly.