Seems to me like the E1 default route doesn't "win."
It gets beaten on both longest subnet match (=1st check) and worse route distance (=tie-breaker)
Since both use same next-hop , you can't tell the difference between:
-default route via 10.3.1.1 being used
-route 10.0.0.0/16 [110/11] via 10.3.1.1 being used
It seems to me like equal cost load balance isn't working, or packets via 10.2.6.1 don't make it to their destination.
Test: On gateway router , add a /32 static route to a test IP in 10.0.0.0/16 range, using 10.2.6.1 as gateway. Now you can test if path via 10.2.6.1 is working