I have the same question. I have a Watch 3 (One UI Watch v2.0, Tizen v184.108.40.206, S/w R850XXU1BUE3), Samsung Health knows I have a Watch 3, so if the setting appears, then surely it should work? If you need a Watch 4 for this to work then the setting should not be offered. Or does this need a watch update that hasn't appeared yet?
Exact same story here..... I take a reading right before I go to sleep, and one right when I wake up, so I know it's working, it just does nothing overnight and still only works if you initiate it manually.
Same thing with me. I can check oxygen any other time, but the auto check while sleeping doesn't seem to work. I also have a watch 3, along with a Galaxy Note 10.