Where were you buying your cards that let you return them for coil whine? The retailer I bought from would want to test it, I just know they'll say it's ok and decline a return, so I can't be bothered to go through all the hassle. I should have bought from the biggest retailer in the planet who have a no questions asked returns policy!
I recently bought a 4090 Suprim X, it has the serial number in the same place as yours (so new batch) and I've got a coil whine.
I think it's luck of the draw, and also dependant on load. If someone's gaming at over 200fps they're gonna hear it far more than someone playing a game at 60fps.
One thing I noticed that's far more annoying than coil whine is a resonance/vibration from the shroud at certain rpm. I've noticed a fair few videos appearing on youtube. Here's a video demonstrating the noise.
I've found it goes when pushing the shroud. It also seems to go back a few generations with the suprim X, there were videos of the 30 series suprims suffering with the same issue.