Aren't those fields populated automatically for each file type when creating new custom tags?
Yes, they are. But the content of what's auto-filled is exactly what the user would have entered under "identifier" to represent the tag name.
On a custom tag slot, if I type out "MusicBee" as the identifier, the values that will auto-fill on each of the file types will be "MusicBee".
I'm new to Custom Tags, but I'm certain that MB delegates 100% of the responsibility of Custom Tag definition to the user. There are no presets/assumptions on its part.
I already spend hours on end on social media. Might as well spare a few of those to a greater purpose here.