Hi. To be honest, I'm no expert, but I read at https://wiki.openstreetmap.org/wiki/Tag:highway%3Dstop that highway=stop means that traffic has to stop every time, regardless of traffic flow etc., which is not the case here. I'm not sure how to mark "this is the line where you should stop if the lights are red", but it's not highway=stop!

I was alerted because my sat nav said "Warning: Stop Sign" when I approached the roundabout, but there wasn't one.