Maybe Mettaton had spoken out of turn by saying that. That wasn't a normal Sans response, was it? No joke, no flippancy. But still, he's not going to drop it.

"Of course he's fine! Papyrus has a way of dealing with everything in a very positive manner!! But darling, I wouldn't have gotten as far as I have if I were dense. He doesn't have to say something to still express himself about it.

You're family. He missed you! I am fairly certain you take that seriously, as you are a good brother."

Which...brings them to a more uncomfortable subject than any they were actively avoiding. MTT would rather add this one to the pile, for sure. But instead, he just gets on with it.

"By the way. I'm sorry for what happened before. During that dreadful invasion. I shouldn't have been so cross with you."