Hello,
I am using a Vivomove3 as second watch to my Fenix6. Last night I did my first sleep with it, but I have neither sleep data, neither stress neither body battery. But HR was synched properly.
Interestingly those values where shown while weiring the watch.
I did put my Fenix6 back on after sleep ...
I come back from a long discussion with Garmin support.
Physio True Up does not transfer BodyBattery status between devices, neither is any watch recalulating this metric by data received by other devices. (Status: October 2020)
So finally BodyBattery does NOT work if you use more then one device, making the feature pointless 
Sleep data is only recorded by the device which is marked as main device.
The missing "stress" was resolved by resetting the watch.
I recommand everyone to send a message to Garmin via the following link telling them you'd really want the transfer BodyBattery between every TrueUp device, as well as the sleep data -- https://www.garmin.com/en-US/forms/ideas/
