I replaced my Note 9's Rear Camera Module from Samsung Service Centre nearby my home weeks ago, Camera works perfectly fine. No issues till now and no dust is present inside my Note 9's Rear Cam. They take 2 Hours in all this replacement process.
My Note9 Water Resistance is intact (no effect on it), because after Camera Module Replacement, they test the phone thoroughly, including its water resistance, my phone passed that test successfully, bcoz they use "Packing Part" (a spare part to seal the back cover properly)
Thats why they take 2 Hours in all this process. Don't interrupt/pressurize them to repair your phone fastly, be patient and relax at service centre. I was also worried for the first time but its completely fine after I got my phone back. 😊