There seem to have been a lot of these threads on MN lately, where there's a group and then one family are left out. Perhaps all these groups are full of really mean people who deliberately like excluding people, even their very good friends they normally get on with...or...
Perhaps it's ok that you don't get asked every time to everything. Perhaps the dd was a bit fed up with your dd and the parents thought a bit of a break might be good for them. Perhaps they fancied a different mix of people, including some new ones. Perhaps it's ok for these things to be mixed up and not everyone go to everything all the time.
When I look in my own friendship groups, we often go out together but also definitely mix it up a lot too- my colleague is a good friend of mine and we get together a lot with the children, plus parties, plus coffee but quite often she invites a slightly different mix to her dinner parties, and I don't feel put out if I don't get invited every time.
I used to tell my children that people can invite who they want to their parties, and sometimes that wouldn't be them. It wasn't- sometimes their good friends would have a party for three children, they wouldn't be one of the three, sometimes they invited them back to their parties anyway. It ebbs and flows, but you don't have a 'right' to go to someone's party ever!
This six-some sounds a tiny bit stifling, if I'm honest.
Of course, it could be there's something brewing and they have a genuine issue with you or your dd's behaviour, and if so, hopefully if they are good friends they will let you know. But I also think it's fine to mix friends differently and I can't imagine being in such a claustrophobic group that if I didn't invite someone over to an event, I would get defriended/they would strop off a group. I can't imagine this happening tbh.
I would just put yourself back in the group and say nothing more about it!