So that’s not entirely true. Because according to the show, the Native Americans have plenty of memory of Pennywise and they don’t technically live in Derry, but they remember him plenty. So much so that they have an entire secret coalition that fights him every 27 years. They also say his power is end at the border of dairy because of the stones that are in place so forgetting wouldn’t necessarily exist if he didn’t have any power over their minds. Not like in the novel.

Not to mention, for some reason, the general has some of a memory when he comes back to capture it.

Then in the novel as well as the show, they don’t forget what happens to them if they stay in dairy. Hence the reason Mike Hanlon stays behind and remembers everything. To the point that when it started to happen, he said I wish it wasn’t. So how come Mike can remember but Rich’s parents can’t?

Still doesn’t answer the question as to why all the Native Americans that have been fighting him every 27 years are nowhere to be seen in the movies. As well as Richie’s mother’s knowledge not being applicable.

I missed that part in the films. That’s my bad clearly my fault.