Considering Mike’s character in canon, it’s not surprising he’s behaving this way in the new animated series. Seriously, why are people suddenly shocked that “they are murdering miwi,” as if Season 3 Mike doesn’t exist? Instead of acknowledging that he abandoned the party and acted like a jerk, he canonically deflected and made it Will’s problem that he’s upset about boys dating due to his differences. He directly said this to Will’s face, making a frankly homophobic remark in the process, regardless of his intent.
His behavior clearly changed from Season 2 to Season 3, mainly due to his insecurities and obsession with Jane/El. This, combined with the “we are not kids anymore” mindset, led him to try and fail to play the role of a perfect boyfriend. As a pubescent teen lacking emotional tools and maturity, he clearly struggled to have a meaningful relationship and made it everyone’s problem.
In one of the comics from that time when El/Jane got back, they hint that Will already feels left out and that Mike is spending all his time with his girlfriend.
He tries to convince the party to go on an adventure together, but they are not very excited. As a result, Will lies to Joyce, saying he will spend the night at Mike’s, and goes alone. She even explicitly mentions that he hasn’t had a sleepover with Mike in a while, again confirming the effect of Mike focusing on his girlfriend.
I know people keep saying Mike is the one who understands Will best and makes him feel like he’s not a mistake, but that’s the idealized version of Mike from the intimate childhood they once shared. During the rain fight, the perfect image of Mike begins to crumble right there in the garage. You can see the hurt and shock in Will’s eyes. In the van, he’s still clinging to that old Mike, believing he can be that, but if we look at the events of season 4, Will was clearly hurting, and Mike was too self-absorbed to notice.
This cartoon thing does not surprise me at all. I know people will again say Mike is the gayest one, but in reality, all his actions stem from his tunnel vision for El/Jane. Let’s all hope Will can at least avoid another humiliation ritual.