Just a reminder that the ID3 standard says that the field TRACK is a numerical string that allows the / as non-numerical character, the rest should be numbers.
So, in a narrow sense, using letters like "A" and "B" would be invalid data, at least according to standard.
MP3tag does not interpret the standard that strictly - but perhaps your player does.
And another question:
perhaps this is a similar problem as described in this thread: