Keywords with Apostrophes

[erichaas]

Recently, I've noticed that whenever I try to use a keyword with an apostrophe in it, then no matter how many times I write-back the metadata, the file always has a pencil icon indicating metadata needing to be written-back. The only way I've found to fix it, is to change the keyword so it no longer has an apostrophe in it. This wouldn't be a big deal, except that I already have a bunch of keywords with apostrophes in them, and so now if I want to add an old keyword to a new file, I have to change the keyword in all of the existing files as well.

The same also seems to be true for the ampersand.

[Mario]

I add the keywords

Child's Play; Alpha & Beta; Don't Do This

to a file and write-back.
No problems. No re-appearing pen icon. All working fine, no problems at all.

A similar issue was never reported, AFAIK.
Please detail your Edit > Preferences > Metadata/Metadata 2 settings, the file format, the contents of the ExifTool output panel after you write-back (in case there are errors or warnings reported), the log file of that session and provide an image file which allows us to reproduce the behavior you are experiencing.
Run the Metadata Analyst app on the file to check for problems with the existing metadata in the file. Use the GREEN button at the top to copy / paste the results.
Maybe a non-standard setting or the existing metadata in the file is causing this?

[sinus]

If I add a keyword

"dog"

(the " is for me a apotroph), then this works, I can see the keyword in the keyword panel and e.g also correct in the exif tool.

But in the keyword-category, the keyword is showed as

_dog_

I do not know, if this is important, no problem for me, because I do have not keywords with apostrophes, and if I had, I would change them all with help of the Keywords in the category, there this is done quite quickly.

[Mario]

But in the keyword-category, the keyword is showed as

_dog_

I do not know, if this is important, no problem for me, because I do have not keywords with apostrophes, and if I had, I would change them all with help of the Keywords in the category, there this is done quite quickly.

This is correct. The " has a special meaning in category formulas and is not a valid character to use for category names.
IMatch does not allow you to create a category with a name containing " or @ and also some other special characters. This woulds break formulas and other things.

When you import categories or a category is dynamically created from keywords, IMatch automatically replaces invalid characters with _

See Category Names for more information.