You could make a report here suggesting an enhancement.
But what comes into the feed is anything posted on kbin as a microblog (potentially by mistake) or, from mastadon where they tagged a community subscribed to.
It isn't normal threads to groups that end up there.
Just looking at the current feed on my instance (you can see yourself, you don't need to login https://kbin.life/microblog) the first item is from a mastadon user that tagged the [email protected] group. Hence why it shows up there since the instance takes content from there.
The next 2 items are clearly people on kbin.social that mistakenly (well maybe deliberately, but I suspect not) create a post, not a thread in the [email protected] group.
You can tell which is which, the mastadon users are deliberately using hashtags and tagging groups with @. The others are making a very reddit style post that is turning up in the wrong place because they chose post, not thread.
But I think it'd be a nice option to just show followed people and not groups. Maybe a three way toggle. People, Groups, All.