I would like you to check the other way round:
Use a file that does not show the 2 fields and enter data with the plex editor for this file.
After Plex shows the data, open the same file in MP3tag and check in MP3tag whether the data also shows up.
If not, then Plex neither reads nor writes this data but keeps it only in its database.
The second approach would be to edit all the metadata in MP3tag first and then import it into Plex as a completely new file and check which data gets displayed in Plex.
For the standardized tags there is no other place.