extended view does not show all tags

I have scanned my mp3s for idv2 tags with perls mp3::tag module and I found a couple of tags which are not shown in mp3tag (even not with ALT+T) and thus cannot be edited.
Is that a known problem or bug?

Below is an example where I highligted a found item in red.

Tag list shown with ALT+T:
ALBUM ...
ARTIST ...
8AND ...
COMPOSER ...
DATE ...
GENRE Chlout
LANGUAGE ...
TITLE ...
TRACK ...
YEAR ...

IDV2 tags found with perl:

Attached picture : (APIC)

                    Picture Type- Other
                           _Data- binary
                       MIME type- image/jpeg
                     Description-
                        encoding- 0

Private frame : (PRIV)

                           _Data- binary
                            Text- <!--coloro:#FF0000--><span style="color:#FF0000"><!--/coloro--><b>http://www.cdtag.com</b><!--colorc--></span><!--/colorc-->

Album/Movie/Show title : ...
Content type : ...
Date : ...
Title/songname/content description : ...
Language(s) : ...
Lead performer(s)/Soloist(s) : ...
Track number/Position in set : ...

Search for PRIV on the forum
e. g.
How to manage Private Tags?
Data Provider Field

DD.20090701.1927.CEST

Thanks for the quick reply.
I did not know about the specific PRIV problems.
There are other tags though which are not PRIV and which are also not shown by mp3 tag.
I searched for discussions about hidden tags but could not find anything appropriate.
I also tried the hint which I found at https://docs.mp3tag.de/actions/remove-fields-except but it did not remove the undesired tags either.

Pls. see the highlights in the samples below:

__./test1.mp3
Album/Movie/Show title : ...
Content type : ...
File type : ...
Title/songname/content description : ...
Length : 218000
Lead performer(s)/Soloist(s) : ...
Part of a set : ...
Track number/Position in set : 01
Size : 53...
Year : ...
Terms of use : (USER)

                        Language- <b><!--coloro:#FF0000--><span style="color:#FF0000"><!--/coloro-->Cop
                            Text- yright notice. These materials are protected by United States copyright law and by international treaties. You may use these materials for your personal, non-commercial use only.<!--colorc--></span><!--/colorc--></b>
                        encoding- 0

__./test2.mp3
Attached picture : (APIC)

                    Picture Type- Cover (front)
                           _Data- binary
                       MIME type- image/jpeg
                     Description- 
                        encoding- 0

Comments : (COMM)

                        Language- eng
                     Description- 
                            Text- 
                        encoding- 1

Linked information : (LINK)

                             URL- <b><!--coloro:#FF0000--><span style="color:#FF0000"><!--/coloro-->ttp://www.emusic.com/albums/13101/<!--colorc--></span><!--/colorc--></b>
                              ID- <b><!--coloro:#FF0000--><span style="color:#FF0000"><!--/coloro-->WCOh<!--colorc--></span><!--/colorc--></b>
                            Text- LINK

Sorry, I really don't know what these fields are. If they do not show up at the extended tag dialog, they are not supported by Mp3tag.

An app that can show all (I hope...) existing tags with their declared ID3v2 frames' names (+ other usefull info about the file) is Medieval Cue Splitter. Freeware, many languages; quick installation, no needs restart Windows. Choose Tools > Audio file information. BTW it's really good app for splitting.

EDIT LATER
Now it handles FLAC, too.