Methos doesn't like surprises, which Merlin was. Being an ass to him was kind of a reaction to that. (I told you last night that I go back and forth about if I think Merlin and co were real in the Highlander universe, and I think I finally decided they were. So part of it was being disappointed it wasn't the Merlin Methos knew.) Also, he doesn't care enough to go and really spread the...future/past around.

You must be emotionally invested in the first place, for a breakup to occur, Mr. Kirk. Meaning Isaak would pretend to be disappointed but be a gentleman about it, while not caring at all. He will be slightly disappointed if this means they will no longer be interacting with each other much at all.