I'm not the only one? That's weirdly comforting.
With me it's not really a flexibility thing. It only happens with my left leg, right where it meets my trunk. Bring my knee straight up to my shoulder makes something... I don't know exactly. Pop or click or settle in a strange position. It doesn't hurt, but it's not comfortable. It's especially odd doings kicks from the outside, or bringing my knee high enough to side kick in the stomach area. After a long day of kicking, it feels strange. It feels like I'm dragging around a vestigal bone in my hip. Or perhaps a badly placed bone that didn't settle back in and got flung out of it's socket repeatedly. It's one of the 2 reasons why I dropped crescent kicks.
I've never had a hip injury. My leg is flexible enough to go well beyond the point where it pops. It's never been an issue with my right leg. Oddly, that leg is immensely more flexible than the other, but not as strong or coordinated. I wonder if it's related, but I doubt it.
I wouldn't call it painful. It sounds like the same actions are causing our issues though. My issue hasn't gone away. It's just more irritating the day following training and then it recovers.