Regulars on the OnePlus subreddit originally guessed that the device's screen wasn't quite mounted the usual way after being able to replicate the jelly effect on other phones when they're flipped. Sure enough, XDA found proof in the phone's kernel source code instructing the display controller to compensate by 180 degrees. Upon looking up teardowns of the OnePlus 5 and the older 3T, Reddit user Tasssadar found images proving that the 5's display really is in an inverted position:

As 9to5Google said, the manufacturer might have decided on rotating the display, because it fits better that way on the new phone. The screen attachment module on the 3T has been taken up by the 5's camera sensor, after all. We've reached out to OnePlus to confirm whether it rotated the phone's display by 180 degrees on purpose. But its earlier statement clearly suggests that it's not open to fixing the issue, or that it's treating the effect as an issue at all. Here's the statement in full:

"The OnePlus 5 uses the same level of high-quality components as all OnePlus devices, including the AMOLED display. We've received feedback from a small number of users saying that at times they notice a subtle visual effect when scrolling. This is natural and there's no variance in screens between devices."

If you notice the behavior on your phone -- not all OnePlus 5 owners will -- you'll have to decide on your own whether it's big enough of an inconvenience to get a refund or to swap it with another model.