I studied in an IB school and my final grades were based on internal exams of the IB courses. I got the diploma but there is an 8 point drop… Will I get rejected from US universities this way?
Nobody can predict what any particular university will decide with respect to any particular applicant.
Generally speaking, it is rare that American universities revoke an offer of admission due to falling grades, but often re-told to scare graduating seniors into studying. It happens more often that an applicant will receive a sternly-worded letter, maybe requesting an explanation for why your grades dropped. (Apologize, take responsibility, re-iterate your commitment to work hard in college.)
If you received a scholarship that’s tied to your high school GPA, that may be in danger.