Since your Band is newish (6/6), has it been updated to the latest firmware yet? It likely has but worth checking. The sleep detection only came in an update.
And the update bringing auto detection should not have disappeared or quit functioning. But a phone soft reset didn't change anything for me. Has anyone else tried the soft reset or deleted the MS health and reinstalled or un paired and then re-paired the band?
I've had mixed success with the auto tracking but atm I'm up a few times in the night with baby business.