I am working as a DJ and I use Recordbox version 7.0.9 and there is a MP3 Tag field named "Remixer"
With this field I differentiate the different mix names like extended or radio edit...
Unfortunately I am not able to make this Tag available in MP3Tag
Maybe it is just a wrong mapping of the MP3Tag but to be honest I don't know how to fix this
Get a file from which you know that it already has a tag field for remixer.
Open the extended tags dialogue to see the name for the field.
Customize the file list and/or the tag panel according to the documentation
and add an object using the field name from the extended tags dialogue.
Here is a list of supported standard tag fields, perhaps MIXARTIST is the compatible field.
Other programs my store such information in their own database and not in the file's metadata. Which would mean that the field is not present in the files and also it could be that the player does not read that field from the metadata.
You would have to try what works with your combination of tools.
Do you have APE tags in the file(s) but you do not read and/or write them?
It could be that the update is already in the file(s) in the ID3 tags but not in the APE tags.
If you read APE & ID3 then APE overrules the display of the ID3 tags.
Switch off reading APE tags as a trial.
Just to clarify for completeness: it was a mapping for ID3v2 from MIXARTIST to MIXARTIST which is not active by default when the app is installed, but which caused the field to be no longer visible.