It seems like happen on your live site, so I was unable to test on your existing group regarding this issue, so I created a new group here ->https://www.yourDomain.de/gruppen/187-testgroup-pleaseingore and tried to comment on the stream, it seems it will not show that comment on the existing stream in the group.
You can take a look of my attached screenshot below.
Perhaps I misunderstand how to replicate this, please advise.