He might flip them, but Lan Wangji keeps his grip like a constrictor - he has increased his upper arm strength since last they'd fought, so naturally he finds it easy to hold on, even if Wei Ying pulls a little trick like flipping their positions.

He bites his tongue to keep from groaning, and with his famous restraint, manages to avoid bucking his hips upward as he distractedly closes his eyes and tries very hard to push certain images out of his head.

On the other hand, he doesn't have much leverage but tries to make a little gap between them so that he can flip back over on top, though without anything to push or kick off of, it might not work out for him.