Perhaps Joel wants to make sure Dixon knows who the boss is? Magnus was really good with Zoey when she first came home. But now, I notice that he's not as tolerant. My niece's dog is still at that puppy stage and she's more playful than Zoey ever was. She gets in his face a lot, trying to get him to play but he doesn't quite appreciate it. He'll put up with it for a bit but not too long. Funny thing is, my neighbor has a golden doddle puppy who's playful as well. But Magnus will not put up with him even for a bit.
I think you should let Joel and Dixon figure things out. Of course, while keeping an eye on them to make sure it doesn't get serious.
Perhaps some playtime out in neutral ground together would speed things along?