Most likely limitation of virtual environment itself, long lasting issue, not able to make mcast underlay work since 9.3(x) versions.

Need to be checked, but can assume Nexus doesn't send BFD with code 7 Admin down (either something else, or nothing at all instead).

This is working as expected.When you manually shut down BFD (or subinterface in this case), it sends a control packet to the peer with Diagnostic Code 7 (Administratively Down). This tells the remote router: "My administrator is just turning off BFD....