Ticwatch pro provides heart beat for only part of the night

Ticwatch pro provides a heartbeat for only part of the night. When I get up in the morning and look at my report the graph shows the heartbeat during the night except for the last 2 hours. What is really odd is that several days later the data for the missing hours is often supplied.

My phone is a Samsung SM-G970U. The phone is less than a year old and I make sure it is acquiring all updates. It is charging during the night. I always make sure my watch is charged to at least 90%. I always have plenty of battery charge left in the watch in the morning. Just before going to sleep, I turn on the Sleep app and turn off the watch face light from turning on. I do this by swiping down the watch face and then pressing the watch icon. In the morning I then turn on the watch face light for about 10 minutes before turning off the Sleep app.
Version of Sleep as Android: 20200828