Hmm. Tricky to tell. It seemed like it was happening regardless if I was toggling the Rim Light off and then on, or having the Rim on and turning the Fill off and then on again.
However, I may have solved it, though it could be short lived. 
I’ve set my render cube to 0, the Key light to 0; I’ve turned off all channels for the Sky Sphere in the scene (probably makes zero difference, but it doesn’t need them on anyway); the Rim Light to 1, the Fill Light to 2; and the model has all three channels on - 0, 1, 2. So far, it appears to have stopped. I might be misremembering (I’ll have to check one of my previous posts), but I may have had more than one channel assigned to the Key Light…? Maybe that was causing it? Like some sort of conflict. But we’ll see.