Not so strange when you look at the details.
First, he only played 2 adult matches so those aren't used and he get a mixed exclusive rating from the 4 mixed matches. And yes, he lost all 4, but they were all close, including 2 where he won a set and was even or just -1 on game differential.
But the kicker is that he was playing 8.0 mixed as a 4.0 with a 3.5 partner. Given this, and especially if the partner wasn't a strong 3.5, they were likely supposed to lose these 8.0 matches, probably by more than they did. So their very competitive results against likely stronger opponents indicated he was playing better than his rating and he got bumped.