Except he hasn't
He ran in to save a group of babyfaces from a heel beatdown in his surprise return on 8/10. Hardly a heel move.
The next week he came out, cut a promo, and ultimately went into business for himself with a stupid remark about Page. But the fact that Punk is a moody, abrasive prick in his real life and has no hesitation to use opt for that "worked shoot" bullshit into his storyline promos doesn't mean he's been a heel in storyline.
The next week he lost a title match after getting "injured" and did a big sympathetic injury exit. Not a cartoonish stretcher job like MJF did.
It also just so happened, through no fault of Punk's own mind you, that Khan opted to put the interim title on a babyface while Punk was out. That wasn't a booking decision that had to be made. If Punk doesn't get hurt, his first challenger in Chicago of all places was almost assuredly not going to be babyface Moxley, right?