From my experience with HW2.5 + EAP, I've gotten better stop sign warning success at OSM-mapped stop signs, but I've also seen the warning for other places too. Looks like there either aren't a lot of stop signs around here or they're not well mapped on OSM:I have verified the Stop Light Warning but have not been able to get the Stop Sign Warning?
I would suggest going to an area that has a high concentration of these blue circles and test at 4-way stops (or at least a street where you can keep going straight as otherwise Autopilot will panic seeing no road ahead).