Both over- and undercooked beans from a pressure cooker I cooked pinto beans in an electric pressure cooker and ended up with a few overcooked mushy beans, few cooked ones, if slightly al dente, and the majority of beans so severely undercooked that they seemed to be hardly cooked at all. Over- or undercooked beans wouldn't have surprised me, but both at the same time? What happened?

I used my newish electric pressure cooker to cook dried, not soaked, pinto beans. I cooked 250 g (9 oz) in 1 litre (4 cups) of water on high pressure for 22 min (the accompanying booklet recommended 22-25 min) and did a quick release (I did 24 min and natural release before; this resulted in beans so mushy you cannot eat them as anything else other than puree; internet research pointed towards 22 min and quick release). Where was my mistake?
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.