I've had two batches in the last year that have had some similar characteristics to what you describe. The beers looked, smelled, and tasted great until the very end, as I swallowed. I described the flavor as "earthy," "dirty," or "dusty." People perceive and describe flavors differently, and I wonder if what I tasted isn't what you describe as "extremely bitter" and "hay like."
After a whole lot of research, I finally came across this excellent reference, which links dusty off flavors to mold (see the Musty entry). I wracked my brain trying to figure out how I could have gotten mold in my beer, coming up empty for a long time.
Then, one fine brew day after my second encounter with this off flavor, I decided to take extra precaution when preparing my Better Bottle to receive the wort. I did a visual inspection before my normal application of sanitizer. I had to look very, very closely, but there it was. A tiny little spot of white mold, about the size of a pea. Being white, it did not stand out and was not easy to see even when I was looking for it.
It is recommended to lay a Better Bottle on its side after washing it because supposedly it will dry faster that way. I did that, and the mold grew in the spot where the little remaining water pooled. After I found the mold, I inserted a wash cloth and gave it a good washing just prior to sanitizing and adding the wort. The beer that fermented in it tastes fine.
So what I'm suggesting, in my very roundabout way, is to check your fermenter(s) very closely for mold before you put your beer in there. Even a tiny bit can have a huge impact.
As for me, washing and then sanitizing my fermenter is part of my brew day routine now. I still haven't figured out how to dry a Better Bottle more quickly to avoid the issue altogether.