I always get confused with this but I was thinking Paris - stopover, Milan - destination. You're right - I forgot the open-jaw point. But still concerning that a stopover didn't price as expected.
What route did you try with a stopover and open-jaw?
Actually you're right, it was me who was confused... you had a stopover and destination, not a double stopover. Mondays, amirite?
I tried ORD-CDG (stop) FRA (OJ) DUS-ORD and it didn't work.