A few chapters ago, the grandma had the mushroom as a souvenir from the time she was saved.
Hmmm yeah that's true, well then there are two explanations I can think of if it really is time travel without alternative reality involved
#1 the government covered up every monster attack as just being a movie and also the monster attacks stopped for some convenient unknown reason.
#2 everybody is extremely chill with the fact monster attacks happened (this is unlikely because they always refer to her as an actor and her grandpa as a director, and talk about special effects and such, so they obviously don't think the monsters are real)
And then if it is alternate reality time travel, the mushroom could just be for a different reason, maybe she got bored while sleeping in the cabin and decided to pick a mushroom, and then later kept it as a momento from that time.
But why I think something other than just time travel is going on is for a few different reasons, such as her clothes never staying damaged, her clothes growing in size to fit her, the events never being discussed as if they are real, the earbuds working despite the fact that anna shouldn't even be born yet, the films showing up as soon as she gets back instead of already having been there 50 years ago (what I mean is that if it happened 50 years ago, why doesn't the present day just change to reflect what happened back then? Why don't the films appear 50 years ago when they were made?) and when she travels back in time, time still passes in their present day, despite the fact that she leaves and returns to the same place.