I just find it strange that he didn’t invite me, and that he’s going to the biggest event they supposedly have. should I be worried?
No not really, he's just trying to reconnect with family a bit, if you were with him they would be more instigated to repel him, that's the way this cult effects people.
Unfortunately any niceties expressed by his family will only be to draw him back into the cult, it will not be truthful, honest by what most people may think. ........ a lot of fake smiles.
Don't take it personally, like he's not proud or ashamed of you.
He left a highly controlling cult that will shun or disown their own family members if they leave their religion.
From a person who doesn't know or how this cult operates and how it effects people's behavior, its hard to grasp why people act or behave when they're confronted with past members.
Take care and let your partner precede with this meeting up with his family without emotionally turmoil.
Thinking about it why would you want to meet with people that are just going to shun or nose you up anyways ?, you to them are dirty unrighteous along with your partner.
That is why this particular cult is so damaging, cruel and socially ruinous to people who get involved with it.