I can't do a screenshot, sorry. I went to the action menu and created a new action. I selected the field ARTIST. I asked to replace / with , . Nothing more. I tried to see if it worked with the case-sensitive option, but no.
This is very true, I didn't notice that case change on my first look.
What I was noting was the desired Replace was to change from "/" to ", " still ended up with "/"
The resulting "/k" in this case versus expected ", K" is even more bizarre.
@bollemanneke A screenshot with the Extended Tags window of this file would be very helpful to provide a solution. Perhaps you have a mix of ID3 and APE tags, or your Action is mixing up one field for another?