I have been grinding organic hard red winter and spring whole wheat berries off and on for over 30 years. I have never had that taste. But buying flours off the shelf in markets I have had it plenty of times. It could be rancid. The bitter taste is charactoristic of rancidity. This is usually accompanied with a slight burning taste after swallowing. I usually taste the raw flour after grinding to check for that. I had to throw away several batches of just baked corn bread from store mixes that I didn't taste before hand. It's an unmistakable taste even after cooking. A few bags of ww flour have been rancid as well. Most people don't use that much whole wheat flour so it sits around in the store too long.

Ron