Dipper and Mabel's parents didn't go to their kids' birthday
Dipper and Mabel I think have their birthday on their last day in Gravity Falls, we didn't see the parents at the twins' party either. That always made me wonder if they're gonna be present in any of their kids' birthdays going forward since I'm sure Dipper and Mabel will want to celebrate their birthdays in Oregon from now on. Eventually, I realized that they probably celebrate when the twins come home after the summer is over. It may not be on their actual birthday but it's better than not being there at all for it.