I have had this issue for a while and have somewhat narrowed it down. Any time I lose Bluetooth between my phone and watch, when it reconnects it starts sleep tracking on both the watch and the phone. Usually it’s because I walked out of range of the phone and sometimes I’ve accidentally turned off Bluetooth.
I’m on release 20181003 but this has been going on for a long time. It was an issue on my S7 edge and still an issue on my Note 9. The watch is an Samsung S3 with the android gear plugin.
This happens a lot during the day at work and has caused many missed calls and messages as it enables DnD when sleep tracking.