Yeah, I know it gets AC, and the poor problem text is why I said "I'd say" it fails that case. I'm pretty sure the problem is consistently using prefixes of the arrays, i.e., we're supposed to use the first m values in nums1 and the first n values in nums2 and then end up with the merged values as the first m+n values in nums1. You clearly do interpret it that way for the input, but for the output you don't, which is inconsistent.
Edit: I added assert m+n == nums1.length; to your code and it still got accepted (and with < instead it didn't, showing that assertions work). So the problem text saying nums1's size is "greater or equal to m + n" is bad, or the test suite is bad because it doesn't actually have such a case.