Why does cooking mushrooms longer alter the taste so much?
I can't speak to specific chemicals/aromatics for mushrooms, but this kind of thing does happen with all kinds of food. The initial cooking is releasing aromatics that were previously bound up in the food somehow, so you quickly get some stronger flavors. But aromatic molecules are volatile (that's why you smell them!) and also generally more prone to breaking down, so as you cook longer, they escape, or are broken down into something else without so much flavor, leaving you with something milder.

Think about onion, for example. When you first start sauteeing it, there's a lot of really strong onion smell coming off, maybe even enough to make you cry if you're prone to it. If you eat it then, it'll still have plenty of sharp onion flavor. But as you cook longer, those flavors mellow out; going to the extreme, slowly caramelized onions have no sharpness and very little onion flavor.

I think the initial strengthening of flavor may be stronger with mushrooms because of their structure. They seem dry at first, with all the water bound up inside. But as you start to cook them, their structure breaks down and they quickly release a lot of water, and with it, those flavors.