I'm the child of parents a bit like these. My parents didn't divorce until I was 20ish, and they've been divorced for 20ish years now but are still very close. Less close now that all their kids are adults (they no longer have family vacations together), but we still have family holiday celebrations and my dad still sometimes helps my mom out around her house. They call each other at least once a month just to chat and catch up. You get the idea.
I can totally understand why it makes you uncomfortable, but seeing it from the inside, I can confirm that some people are just better friends than lovers. My parents were both very unhappy when they were married. There were no big fights, no obvious dysfunction that anyone else would notice. They just got married very young and then grew up into two very different people, and while they tried to compromise, the compromises left them both miserable.
And yeah, they both had trouble dating, because a lot of people are uncomfortable with their relationship. But they're both remarried now and their spouses are a part of the family holidays and stuff. And I have no doubt about their priorities. Both of them put their current spouses first over the needs of their co-parent...as long as that basic respect and understanding that it's a package deal is there. Their current spouses are both very lovely and get along well. My mom even worked with my dad's wife to redecorate her house, which involved them spending a huge amount of time together as friends.
What you've said gives no indication that you should be concerned. His ex even asked him to cool it a bit when he was spending too much time with them. He's open about his priorities, and he's good friends with his ex's spouse independent of his relationship with her--this isn't exactly her keeping him on the backburner and him trying to sneak his way back into her life.
But it is unusual, and it's okay to not be comfortable with this. It goes against a lot of cultural messaging about what "normal" divorced couples are like (even friendly divorced couples). But I know I'm not the only one with parents like this, because I've met others. And frankly, I think it's quite healthy as long as he's not prioritizing his relationship with his ex and her husband over you (with the caveat that he has to prioritize his co-parent relationship with her over you, for the sake of the kids--but I'm talking about their friendship beyond that).
So what you should do? I don't know. I'd advise starting by talking to him and expressing your concerns and discomfort in an honest but non-accusatory way. He's not going to be totally unprepared for this, since at least the one previous relationship ended because of it. See if you guys can come to a place where you're comfortable. If you can't, then maybe you give it more time to see if you can get there, or maybe you move on. I don't think there's really a right answer here, so that's something you'll have to decide on your own.
Definitely don't contact the ex though. There's no way that goes well.