While this data field isn't officially supported for this device in the SDK, it was working fine on third-party watch faces until the last update. It now appears to be missing, and the same seems to be true for VO2 max data for both running and cycling.
This is a bit strange, as this feature is available on the Vivoactive 5 and Venu 3, and it was working fine on third party faces until this recent change.
