Small bump in my 15" MacBook Pro 2018
Here’s how I would try to figure this out.

if you have a straight edge, perhaps a 12" metal ruler, try laying that ON EDGE across the keyboard at the bottom, top, and middle, and perhaps even a couple diagonal angles. I'm not sure if the newer models are like mine at home, but the keys rest just ever-so-slightly below the case, so you should be able to see if anything is approaching the ruler's edge across the span.

if you're unable to tell, something like this could probably warrant a pic or two added here, or even if just taken from your phone, which could then be examined by eye and perhaps by selection in a photo editing app. it wouldn't be 100% accurate, but if there was an upward shift of pixels in the middle versus the two ends, you might have something. 

either way, better to be sure now while under warranty, so you can at least get it on notice in case it gets worse. if it's related to heat, or perhaps some sort of battery swelling, it might be more noticeable at certain times. I've had several batteries expand over the years, and I believe they all stayed the bloated width, but before they became damaging or even just noticeable, they might have swelled and reverted, I can't say.