Asked in Support but got no informative reply.
Why not to support relative paths in /fp? It would allow to start MP3Tag with /fp:"." option from a console file manager (FAR, Total Commander etc.).
If MP3Tag needs a full canonical path for some parsing, it could use GetFullPathName function.
In 2.80 (and maybe some next versions), it was possible to use the "mp3tag .\" form. After upgrading from 2.80 to 2.91, such form no longer works, as well as /fp:"." or /fp:".\".