I just tried to recreate the behaviour to see 2 backslashes in the year field.
The only way that I succeeded was to create 2 fields of the type YEAR in the MP3 file, copy the tag and paste it to the MP4 file - in both cases, 2 fields of the type YEAR are now present - and visible in the dialogue extended tags (Alt-T)
Could you check the MP3 source file for integrity with one of the tools liked here: