Seth never turned, the heels turned on him.
There was a writeup on Reddit about how Seth still thinks he's the same guy but the crowd "knows" better, feels for him because of the knee injury, etc.
He's a shades of grey character in an era where WWE either books faces to do heel things just because (Roman started the feud with Rusev and kept escalating it) or they book heels like faces (it took Jericho 2 months to get boos for the list even though everything he did with it was heel work).
What else could Stephanie give Seth besides a title shot anyway?
I don't disagree with you on the writing not being clear, but the vagueness allows us to fill in the blanks a bit. I'm ok with that, in this specific instance.