Does anyone have watch DND exceptions working?

I'd like to use my Samsung phone and watch in the following manner: phone never makes any noises. Watch vibrates on configured notifications during the day. I have this working for years by turning the ringer volume to "silent" on the phone and configuring the watch to vibrate only. So far so good. Next, I want the watch to not vibrate at night. I also have this working by enabling "Sleep Mode" on the watch at night. All good. Now the problem: I want certain app notifications (Slack in my case) to bypass the Sleep mode DND setting. There is a way to configure DND app exceptions on the phone, which i have done. But it doesn't work. No vibrations from the phone when Slack notifications arrive during Sleep mode. Does anyone have this scenario working? I'd be very grateful to learn how to configure it. Note that I've tried both combinations of "sync mode with phone" and followed the setup recipe as set forth by the Google AI.