Update: It's been a while so I thought I'd follow up with a few more things I learned looking into this further.
While the issue is still present, I think the problem is that the skin is for some reason using Grendel Prime's attachment scheme instead of its own. Going over Grendel's other skins again, they do not have this issue. The attachment issues noted are exactly the same both in placement and size as they would be on Grendel Primes default skin. Because of this, I decided to look into more Voidshell skins. I've noticed Saryn shares the same issue (albeit it's more subtle), while Nidus, Wukong, Mirage, and Mesa are fine and correct. Others were just too similar with their base skins to notice anything, or don't have Primes to even have an issue like this to crop up. And yet Rhino's Voidshell was unique from its other skins slightly (weirdly).