That's not displaying correctly. It should be outputting as follows.
commented on [type] [title] in your group [group]
So for example it'd be the following.
commented on photo grafik.png in your group Bug
Looks like there's a bug with one of the comment notification logging system actions. The asset is wrong. The Activity, Comment, and Notification entry should all have an Asset of "groupjive.group.GROUP_ID.gallery.photos.PHOTO_ID". It looks like the notification is logging with "activity.ACTIVITY_ID", which is not correct and is why you're getting the odd notification title.
Have added a bug ticket to investigate further as it's probably an issue in one of GJs notification system actions.
forge.joomlapolis.com/issues/8241