I’m not saying it can’t happen, but I do think it’s really rare dust all of a sudden can randomly appear under the camera lens cover when there was no dust present before. I would say approximately 90% of the cases that I have read about, the dust is trapped under the lens cover because of the manufacturing conditions when the iPhone is being assembled in the factory itself. Rarely, if ever, Have I ever heard of dust randomly appearing under the camera lens cover, unless the dust somehow managed to move (Which I doubt because of lack of air under the lens cover), or; you have a gap somewhere in your iPhone that is allowing dust/debris to enter (Which is what I am inclined to believe). That would be the only other logical thing I could think of.