gillian probably cancelled her appearance in maine because she’ll be filming that kate mckinnon spy movie. she might not be on set straight away for season 11. when iwtb started filming, she joined 3 weeks into the shoot yet it didn’t affect the amount of screen time she had.

It’s different for a series than for a movie though. Usually, they shoot one episode a week. So if she misses the first three weeks, it means she won’t be in the first three episodes. Or barely.
It’s possible that she has filmed her scenes already for this movie. I don’t think her role is big, and she has kinda disappeared for a week or so. Maybe she was in Hungary in the beginning of July.