I purchased it directly on Kayak and there was no booking class on my receipt. I believe the Y-Class shown on my MileagePlus summary is the actual booking class, not just the cabin. Y-Class is full fare economy and earns 100%.
I have an unrelated Turkish Airlines flight on there (IST - FRA) that shows up as V-Class where I was properly awarded 100% mileage credit. So I'm confident that the Y-Class shown for the first leg of the itinerary is the actual fare class. My question is if it's possible to buy a round trip ticket with just two legs and have them ticketed as different fare classes.