When I paste a good MP3 file tag to an MP4 file, the MP4 year always displays extra slashes after the year digits. I want the MP4 destination year to reflect the same four digits copied from the MP3 file tag, with no added slashes or other characters.
Check the MP3 file whether you have a more than one field of the type YEAR.
Open the extended tags dialogue (Alt-T) and check the YEAR field fairly at the bottom of the list.
If there is more than one, than you can decide whether you want to delete such fields (there is an action for that) or whether the fields should be joined so that they become one (also with an action).
There is only one Year field in each of the source MP3 tag and the destination MP4 tag.
Additionally - pasting tags from MP3 to another MP3 does NOT create this undesirable behavior. Only when pasting from MP3 to MP4, which I do quite frequently.
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: