If someone I'm dating did that to me, I'd want answers.
There were times when I didn't see one of my exes because she was a downer that kept asking for me to get a place for me and her on benefits. I said "if you're like this now, what would it be like if we were living together?"
"What do you mean?"
"Well, you're always miserable about something and it sounds like you just want to move out of your dads, but I have to find a place for us to live?"
Then she'd get angry and ask if I really want to be with her. I mean, I barely knew her when we started dating and she rushed into the relationship using sex to keep me along. I'm sorry, but I don't want to be with someone just to get my own place, or rather, get her her own place.
Anyway, I flat out just broke up with her. I couldn't handle it anymore.
Another ex of mine was actually really nice and good to me, but we broke up due to my mental health and us having different plans. So in that situation, she couldn't handle me anymore.
But it seems odd that he arranged to meet with them without telling you.
Google probably isn't helpful, but...
https://www.google.com/search?q=why+would+your+partner+meet+up+with+friends+with+out+you&rlz=1CADKLE_enGB977&oq=why+would+your+partner+meet+up+with+friends+with+out+you&aqs=chrome..69i57j33i10i160l2.18236j0j7&sourceid=chrome&ie=UTF-8
Anyway, it might be something innocent like time away from you? But that doesn't make sense either because I thought you were friends with them too.
I think he's being a t*at.