Hmm. When I have the curb hooked to the transom beams it captures the stringers in the notches. I have no glue anywhere in the bridge, but I can turn it upside down and everything stays put.
The flexing you mention, is it a diagonal motion within the plane of the road deck? That kind of flexing would drive the stringers out of the notches.
Do you have the diagonal braces on the bottom of the bridge? They help a lot to dampen flexing in the plane of the road deck.