PNG with two comments

[Aronai]

I have a PNG created by Clip Studio Paint with the following tEXt chunk:

Code Select Expand

PNG tEXt (26 bytes):
  [adding COMMENT]
  COMMENT = Celsys Studio Tool

If I do exiftool -PNG:Comment file.png this block is displayed.
If I do exiftool -PNG:Comment="something" I end up with a PNG file with

Code Select Expand

PNG tEXt (26 bytes):
  [adding COMMENT]
  COMMENT = Celsys Studio Tool
PNG tEXt (29 bytes):
  Comment = something

So for the purpose of reading, it seems to consider the (somewhat strange) initial tEXt chunk to match PNG:Comment, for the purpose of updating PNG:Comment, it does not.
If I do exiftool -PNG:Comment= it will remove my 'something' comment. If I do it again, it will not consider there to be any matches for PNG:Comment in the file, another asymmetry from just reading PNG:Comment.

How can I properly update/remove these comments? They are removed if I do -PNG:All= but removing all PNG tags isn't ideal.

Addendum: To clarify, I tried PNG:COMMENT as well. It doesn't seem case-sensitive.

[Phil Harvey]

It seems  that Clip Studio Paint is using an incorrect uppercase tag ID.  (The "adding" message indicates that ExifTool is adding this to the internal tag table because it isn't a standard tag.)

To write this non-standard tag would require creating a user-defined tag.  See the sample config file for examples of how to do this for PNG TextualData tags.  You should give this tag a different name so you can distinguish it from the standard "Comment" tag.

- Phil

[StarGeek]

It seems  that Clip Studio Paint is using an incorrect uppercase tag ID.

The relevant section of the specs on w3.org

     The following keywords are predefined and should be used where appropriate:
<snip full list>
     Comment          Miscellaneous comment; conversion from GIF comment
...
     Keywords must be spelled exactly as registered

[Phil Harvey]

Thanks StarGeek, but you left out the crucial part of your quote:

    In particular, keywords are considered case-sensitive.

- Phil

[StarGeek]

Heh, yep.  I stopped reading at "spelled exactly" part.