Did you ever get the auto detect condition to work? I've tried it for two nights and the sleeping condition has not worked (yet). Bizarre there's no official instruction anywhere.
I'm not sure what triggers "auto detect" conditions on Bixby. I have a sleeping routine set that changes the power profile to medium and turns off the always-on display with an auto detect sleeping condition. Tried it out last night and it never triggered the sleeping routine. I haven't found any information online explaining it adequately.
If it's night, and you put your phone down, then bixby detects it. Based on your sleep timings, it adjusts accordingly. Some kind of machine learning is involved in the detection process