Negative Treadmill Grades Clamped to Zero

I have a negative-grade capable treadmill coupled to Zwift via BLE. This combination does a remarkable job integrating positive treadmill-grade (incline %) over time into elevation. But with negative grades-- not so much. Negative grades always get clamped to zero.

I have examined negative-grade BLE packets coming from the treadmill using a BLE sniffer app and they seem to be appropriate. (E.G. 16-bit signed integers x 10).
Is there anybody who can confirm that negative treadmill grades work on Zwift??? I’m trying to pin down if this is a treadmill-side or Zwift-side error.