Spend one morning last week preparing my home grown baking pumpkins to bake for puree and crisp up the seeds. Once cooked I tasted a seed, it was horribly bitter, then I nibbled on the flesh of the baked pumpkins and they were rancid tasting as well. Unfortunately those little bits caused me to have a bad case of food poisoning.
I live here in Fort Collins, so you know the season we've had, also I've gardened here for over 20 years and often grown pumpkins.
Larimer County Colorado
There are several things that could cause bitterness in pumpkin flesh and seeds ranging from the type of pumpkin to how long it was stored. Ornamental varieties or even volunteer plants that grew from previous crops can have a different genetic composition resulting in a bitter taste. Rancidity can be a problem if the pumpkin was stored at room temperature for very long. You didn’t mention if you grew the pumpkins but this is certainly a challenging problem because there isn’t a way to know about the bitterness without tasting.
Here is a link to information about pumpkins from Oregon State University: