Apple Health Counting Steps While Asleep

I’ve got a recurring problem with the step count on my iPhone X. It keeps regularly counting steps while I’m not moving around, mainly at night when I’m asleep. It doesn’t always do it. It will count the same fixed number of steps per hour until I start moving around again. Although in some cases it does it for entire days, so I can see my activity for that day at all. It’s almost like it stores up the steps across a time period, doesn’t know what to do with it and then averages them out. The pic shows it better than I can explain. I don’t use any third party apps, don’t have an Apple Watch and restarting the phone makes no difference. Any ideas?