Though, something I noticed on IceCubes with that feature, it doesn't work, for me at least. It only uses the first hashtag in the group for me.
The way I figured this out is, when I went to the group, I looked for posts that contained JUST hashtags that didn't include the first one. I'm willing to bet you won't be able to find any. Then search for a tag in your list that's also not the first, and see what youre missing from the list....😬