A: I doubt it’s the humidity that’s causing this, but if you’d like to know more about “acceptable” RH levels, I suggest you read my blog, The 70/70 Myth.
Now to the issue at hand: It’s not unusual for a lot of cigars to turn bitter in the final act, especially some of the more full-bodied cigars which have a higher ligero content. It could be that you’re drawing to hard or too often on your cigars which will cause more juices to build up along the journey.
Re spice: I define “spice” two ways. 1) “Peppery” spice, where your palate and the back of your throat experience something similar to eating a hot pepper. 2) “Sweet” spice, like cinnamon, nutmeg, that kind of thing.
Re other flavors that you don’t taste, it could just be your body chemistry. I’ve spoken to a lot of cigar smokers in your camp. Moreover, we did a survey a while back asking about this, and the majority of smokers said that they do taste these flavors; I know I do.
A stable environment will help your cigars smoke better, but rather than point to the humidity, I would re-think the way you’re smoking your cigars. Other than that, it’s important to note that some cigars can vary from box to box, year to year. If you find that some cigars are a bit “off,” try giving them few weeks in the humidor for a little extra “settling” time. Even that can make a world of difference in the long run.