You didn’t mention if the beans were completely submerged in the water, so if not, please make sure they are.
Second, assuming all the beans were of the same age, I find that soaking the beans evens things out. The speed of the pressure cooker can only overcome so much resistance to moisture in the beans themselves.
I have cooked very old beans that have some beans that never get soft, but that doesn’t seem likely here since your beans are from the same bag.
Lastly, that could have been an odd bag. You might give it another try and see if you have different results.