Can't add exif tag to png image in Unity

[tomekkie]

First of all, thank you for the great job of creating and sharing this library,

I am trying to add a tag to screenshot made in Unity.
bytes = ImageConversion.EncodeToPNG(screenShot), screenShot.graphicsFormat, (uint)resWidth, (uint)resHeight);
System.IO.File.WriteAllBytes(filePath, bytes);
try
{
ExifData TestExif = new ExifData(filePath);
TestExif.SetTagValue(ExifTag.ImageDescription, "description", StrCoding.Utf8);
TestExif.Save();
}
This works for jpg, but not for png.
Then the ImageDescription tag displays in W11 as "title", and in AdobeBridge as "Description".
But I can not read it back.
Finally I have used ExifReader from here to read it back:
(https://github.com/dxball/TestUnityExif)

The set of tags it supports is not complete.
I am browsing images in AdobeBridge and see that a number of commonly used tags are missing. Some of these missing are:

title
alt text
keywords
headline
copyright status

Some tag names are twisted.
Like copyright tag which is not “copyright” as much as “copyright notice”, while the “copyright status” is missing.

I was hoping I could create a sample file with desired exif data and then use it as exif source for ReplaceAllTagsBy(SourceExif) method.
The method is supposed to copy a complete exif data from a given source image,
Unfortunately this fails, because "ReplaceAllTagsBy(SourceExif)" doesn’t copy the complete exif data, but only a selection of tags including these supported only.