Just received my Varia 515 and attempted to connect/pair to my 945.
Radar connected just fine. So I thought I was good and went and connected the Varia to my wife's Vivoactive 4 so she can use it as well. I was surprised when it prompted me to connect via a "light" pairing there as well so I did that and that connected just fine.
When I went back to my 945 to also pair the "Light" mode it fails to connect or see the sensor. I removed all pairings from my wife's Vivo4 in case that was causing issues (it shouldn't but I'm just trying things) and still the 945 flat refuses to see the Light mode on the Varia 515. Radar continues to connect just fine. Not sure what else to do as this seems to be some kind of bug.
Here's the duplicate thread in the 945 forum: