It's obviously a bug. But given the fact that the heart even showed up at all on Apple Watch tells me a watch software update may not be necessary. After all, the "play" glance (or whatever its official name is?) is simply an extension of the Music app on iPhone, just like all other Apple Watch apps/glances are "extensions" and currently bundled alongside iPhone App Store apps.
WTF... just as I was writing this, I decided to try it again. Try tapping slightly ABOVE the heart icon and see what happens...
Yep I'm on 8.4. I get the same options you describe except they're not greyed out. But iOS 9 beta users should be getting an update next week that fully enables Apple Music. You'll likely have to wait for that one.
I assume that last one is the watch (obviously I was only asking about the watch, this being the watch forum and all). So you can't skip and back and forward anymore? Hehe, kind of glad that didn't make it to mine.
Correct me if I'm wrong, but I noticed that when I had the songs in my music already (songs that were purchased previously) I can't click on the heart. I do see that it becomes pale when I click on it, but it does allow me to like it. Probably because it's already "liked" in the backend as it's a song already owned.