Yes, the top of mine is also slightly curved, but the underside is completely flat. However, this curvature (at least on mine) doesn't seem to make any difference: I've been using the 4 TB more or less continuously for a good year now (it's attached to the Mac), and haven't had any issues.
I already have two 1 TB units in use. Now I have ordered another 4 TB. The enclosures are slightly different (could be that something has been customised. The 1 TB's are from 2022). Now I've noticed that the top of the 4 TB is slightly curved. But it doesn't look intentional in terms of design (below the SanDisk emblem). More like: I'm squeezing something big into a box that's too small. Does anyone also have a 4 TB SanDisk Extreme Portable and could check this on theirs?