Halloween Kills' Rotten Tomatoes score was a Fresh 62% just a few days before release, but then it dropped all the way to 38% in just a few days. While the Halloween franchise isn't known for its stellar reviews (only the original and Haloween 2018 are fresh in Rotten Tomatoes), the behavior of the Tomatometer in this situation is still interesting, and a phenomenon that seems to happen more and more on the review aggregator.
The drop in score also dropped the movie a few spots in its franchise ranking, going from the third-highest Halloween Rotten Tomatoes score to the fifth-highest, behind Halloween III: Season of the Witch and Halloween H20: 20 Years Later. While the main Tomatometer score dropped quite a bit, the Top Critic score was already a low 33%, and has since dropped to 28%. Despite the low score, it's important to note horror movies can have notoriously low scores in Rotten Tomatoes, and the audience score is a solid 71%.
As far as the how and why of the score dropping, it can usually be chalked up to the mechanics of Rotten Tomatoes and the studio's pre-screening strategy and the same Rotten Tomatoes score drop can be seen with several other big 2021 releases. Since Rotten Tomatoes distills every individual review score into a simple thumbs up or thumbs down, it can be easy for a movie's score to not fully reflect the actual reviews, especially when the total review count is low and each review has a much higher sway over the total score. As a result, it can only take a handful or reviews to swing a movie from Fresh to Rotten. Halloween Kills had 29 reviews logged when it was at 62% Fresh, and had dropped to a Rotten 59% just three reviews later, and has only fallen more, currently standing at 28% with 154 reviews.
The changes to Rotten Tomatoes scores seen at lower review counts can be especially drastic when only a few early reviewers have screened a movie due to review sampling. Often early reviewers are more amenable to the genre or franchise in question, meaning they tend to swing positive and may not reflect the average critic, in which case, the score tends to drop as more reviews come in.
Franchise devotees don't seem to be too bothered, as suggested by the movie's 71% audience score and strong opening weekend box office. At this point, anyone invested in this franchise probably isn't too concerned about reviews, since the high score of Halloween 2018 was an anomaly for the franchise other than the original movie's 96%, which is still the franchise's highest reviewed movie, although even that score doesn't accurately reflect the critical sentiment at the time of release, since Rotten Tomatoes wouldn't be invented for several decades after the original Halloween came out in 1978.