Show really all mp3 tags & "hidden" genres

Hello all,
my mp3tag has always been set to ID3v2.3 ISO for mp3 files.
And up to a a few days (I am using mp3tag for about 15 years now) I thought that ALT+T is showing me really ALL tags which are written to mp3 files.

But I realized that my Ampache Server showed my some genres in my music i did not know. A few albums (out of my thousands albums) were showing lets say "A". I checked and mp3tag was showing me "B". Even ALT+T was only showing me "B". So I checked them in Foobar which showed me "A" and "B".
Since I didn't know what to do I deleted the genre using ALT+T, saving, and setting "B" again, saving. And then it worked.
I even came across an Album that Foobar says that it does not have a genre at all altough mp3tag is showing my my right genre.
So the only explanation I got in my mind is that "A" has been set as ID3v1 and "B" has been set as ID3v2. But thats only a guess. I hope at least Florian can help me?

To sum it up I have two questions:

  1. please explain what is going on here
  2. is there any way that really shows me EVERY tag (with content) an mp3 file holds (no matter ID3v1, ID3v2 or whatever else). I mean really ALL that is in a mp3 file?

Thank you a lot!

If you suspect yourself that the ID3 tags are out of sync, I would load the files and read first e.g. V1 tags and then V2 tags (can be set in Tools>Options>Tags>Mpeg).
There is a way to see all the tag data but not at one glance.
By definition, MP3tag has a priority for tag versions:
The top one wins.

So you can either delete all tag versions but one or you force an overwrite and save the data again which then writes the data from the tag with the highest priority.