(Unsuccessful) Updates:

Changing the path types between A and B (relative vs. absolute) hasn't made a difference.

I've double checked; not a single XREF is set to overlay.

To clarify: When I try to references the nest, A is found, but B is not. When I load A, B is found. I've had Mac-Windows server mapping issues in the past so it could be something along those lines, but it would be strange if B is found in some instances and not found in others. Additionally, a third XREF nested within B (we'll call C) is found even when B is not. So AutoCAD is finding A and C but not B.