It’s not that it wasn’t properly sealed to begin with, it’s that the dust in the factory conditions during the manufacturing was able to get trapped under the camera lens cover/module area. Once Apple cleans the camera lens cover area, inspect it before you leave the store, to Avoid any concerns.
I am not sure I can agree with you. My iPhone X when I bought it in November it had the 3 cameras totally (I mean, TOTALLY) clean. After 5 months I have discovered last week a white dust dot in the front camera (outside of the objective circle) and also in the rear 2 cameras (also outside of the objective circle, sticked to the glass covering the full camera body), both have dust that didn't have before. So my 3 cameras are not clean, something that never happened to my original iPhone, iPhone 4, 6, 6S and even the 8 I have (work phone). Or even all my iPads.
So if it is factory dust (pretty sloppy if it is), it took months to move an appear in my 3 cameras. I am into believing my iPhone is not properly sealed or there is some small entry between screen and body or something similar. Suddenly my 3 cameras get dirty after some holidays with the iPhone for the first time in a pants pocket for hours. Btw, I have an Apple iPhone leather case, which tends to collect dust next to the earspeaker for calls...
I am considering going to an Apple Store in Berlin or not, as I don´t think they have a "dust free" room, so the chances I get more dust in during the repair, and more dust coming in the next weeks after the repair, high.