There is definitely an adjustment, seeing nearly every pitcher who can spin the ball for strikes is much tougher than being able to spit on HS breaking balls, watch that looper come tumbling in or only sit fastball. but I do think it's somewhat about being exposed as maybe not as good a hitter as they think they are. Not a complete hitter yet vs it just being about better pitching.
I think college bound HS hitters can feast on bad or sub quality pitching in HS, but I also think good college bound hitters, who understand how to have an approach at the plate, will hit at the next level. By college bound hitters I mean the kids who are perceived as good hitters because they hit the bomb now and again but maybe hit .300 (in High School), vs the good college bound hitters who hit for average, power and in clutch situations because they know how to read a pitcher, the situation, pay attention and make adjustments. They might not rake like in HS right away, but they can have success. I have two examples of this at the DIII level, one a player my son played with in HS, who started in Center in college as a College Freshman and was awarded multiple conference player of the week awards this year, the kid could just plain hit. The other is a teammate of my sons who as a College Freshman won the DH job and first PH off the bench because of an exceptional approach, he spent the majority of the year hitting well over .300 with multiple clutch and game winning hits. You could see him making at bat or next at bat adjustments and was an exceptional situational hitter (plus he has power).