Wind Down disabled after 1 minute

Detailed description of the problem:
I have Android’s Digital Wellbeing Wind Down feature set to automatically activate between 11PM and 6AM. This sets my phone to Do Not Disturb and makes the screen greyscale. When I start a sleep session with Sleep As Android, after about 1 minute, it disables Wind Down mode. It only happens if I launch Sleep. It happens even if I place my phone on a table in a quiet room, so doesn’t seem to be reacting to anything. Note: “Delayed sleep tracking” and “Do not disturb when sleeping” are both disabled.

Steps to reproduce:

  1. Ask Google Assistant to “wind down”. (Fastest way.)
  2. Start Sleep Tracking (either in-app or via a widget, both produce the same result).
  3. Wait for about 1 minute. You can also use the phone during this period, it makes no difference.
    Expected: Wind Down mode continues.
    Actual: Wind Down mode ends.

Version of Sleep as Android:
20200330 on Android 10 on Pixel 3.