How to create a discussion that is shown only in a certian group (not public)?

We use groups and spaces in our site. We have been adding "discussion" and "posts" that are visible only in the assigned group/space by using "add to context" bock (see image below). However, recently, for some reason (someone must did something) when we post in the group using the same (add post) field, all goes to public and not to the context. How to fix this?