Just spotted this YouTube video by ALLSTARSPACE a few minutes ago.
Today I can’t get the background activity to toggle anymore. It is just gray. I was half asleep and hit the toggle now I’m stuck 
With my Galaxy Watch 4 Classic it has always worked, but with my GW4 “Active” it has always been greyed out and I’ve never been able to turn it on. Fortunately for me I never use AOD except for testing a watch face.