It seems to be broken, because the author information row uses the same CSS classes as the inline mentions. (width 100%, etc.), see Nextcloud 13 for help how it should look
cc @nextcloud/designers
Avatars are going to be removed from mentions in comments (and then I guess that we should remove them from the activity view too) so that should implicitly fix this issue.
Okay, so let's keep this issue, to make sure this gets addressed before we release something.
Looking at the Riot redesign, the avatars in mentions look pretty great:

Sorry for picking this up again @nextcloud/designers (especially @pixelipo ;) but it seems we just need to find the right adjustment – like with putting a container with slight grey background around the avatar and name – and then it looks great.
Fixed by #10288

Most helpful comment
Looking at the Riot redesign, the avatars in mentions look pretty great:

Sorry for picking this up again @nextcloud/designers (especially @pixelipo ;) but it seems we just need to find the right adjustment – like with putting a container with slight grey background around the avatar and name – and then it looks great.