Why Are Topics Getting Posted in Private Groups?

Apologies - I did a search but didn’t see answer to this one.

We have several private groups. Each group has a private category where they can post topics of interest to the group. We have the perms set up so only each private group can see/post to its private category.

We thought that the group message function was there to message members of that group. For example, to have private discussions.

So, we were surprised to see topics posted in public categories by group members getting posted in the group activity list (as a post or a topic). This was unexpected behavior.

Is there a way to turn it off this automatic posting so the Group Activity is only displaying messages sent INSIDE the group by group members?

Thanks!