I am having this same issue first reported to Apple in Nov 2019
I keep getting random times and days two pop ups for 2 unknown apple IDs
They always appear on the watch first and then when I dismiss it in moves to the watch. I have set the watch as new and even restored the phone as new. I have no music/videos that's why we have Spotify and netflicks for. I have spoken to different apple people and they all keep telling me the same. "only way is you logged in what that Apple ID and downloaded something" No I didn't I don't know these Apple IDs or password.
If the popup was weekly I would get over it but it normally pops up twice a day and sometimes in will pop up, up to 6 times on both devices in like 5 mins, it annoys me so much I reset the watch. Some times it doesn't bother me for a week. I have no idea when it will happen. so random,. I spoke to apple again today and the person was also struck as she knew I was techy so I knew what I was on about
Both my devices are up to date as of this post
APPLE PLEASE FIX THIS OR MAYBE TELL ME WHAT IS CAUSING THE POPUP :(