Comment 7 for bug 47379

I tried to work around this problem by means of the "metric" option in /etc/network/interfaces but I did not succeed. The smallest value allowed is 1 whereas the metric assigned by NM to the route to the default gateway via wlan0 is 0.

I suggest that NM assign a value higher than 0 to the route so that it's possible to use the "metric" option in /e/n/i to give a lower metric to a route via another interface. The value 100 (ifup's default) might be a good choice.