I've never had much in the way of useful success with the GroupCount() function. But I'm not really sure what you are trying to achieve here either, so some more information would be good.
First, do you really mean you are using a Category View, or are you talking about adding Categories under a Panes View?
The example MrC gives here definitely looks like the column in a Panes View:
https://yabb.jriver.com/interact/index.php/topic,81766.msg564675.html#msg564675 If you are really talking about a Categories View, I'm not sure where you expect the count to appear. Based on my research GroupCount doesn't work in Thumbnail text, although that has been requested at least once, and I didn't do a thorough search to see if anything was done about that.
I can see what you mean though. I built a simple panes View and restricted it to [Media Sub Type]="Movies", then added the Categories [Genre], [File type], [Name], and [Actors]. I put a GroupCount on [Genre] and [File Type], because those made counts sense. A Panes View puts a count on the header row for each Category/Column in the View anyway, so I got total counts for all categories, and counts for each group in [Genre], [File Type], and [Name] but no counts for the [Actors] rows, because each of those would be (1) anyway. As you can see, each Movie [Name] gets a count of (1). The Actors column nicely listed each actor in the movies selected via the higher (left) Categories. It does that whether I select one movie or all movies. See the first attached image.
Then I added GroupCount to the [Actors] Category and I see what you mean. The Actors field now gets treated as a String variable, and not a List. I even tried forcing it to be a List with the &datatype=[li
st] function, but that didn't work. Note the correct count at the end of the Actors list though. See the second attached image.
Is that what you were trying to build?
Is that the problem you were seeing?
If so, my question still is, what were you expecting to see, and how would it add value to what is in the View in in the first image?