Marius appears to be only giving a limited set of dialogue. I can’t ask him about my other companions, for instance, and talking with him does not resolve the (!) notification I have right now.
I noticed this was a bug after completing the assassin quest, but I believe it started before that. There was some (!) dialogue that did trigger after that but nothing to do with what happened with the assassin. After that, I was able to ask about my other companions (which is how I learned it should be an option in the first place), but talking with him again didn’t give me that option. Also, there’s still a (!) over his head but nothing triggers.
I’m playing on the original Steam Deck (non-OLED).