Happened to me when I saw a book on the US site, I wasn’t able to see all reviews, but there was a button to sort-of “apply” to get approved for seeing all of them.
It took five days, then I got this email.
Interestingly, they’re not doing this on the European site, you can still see all reviews there.
For what it’s worth, I’m in the US Amazon app on Android and it states “96 customer reviews”.
Once I unchecked the box requiring verified purchases, it does in fact show 96 reviews.