We only support general nonlinear edge-edge contact for two line bodies that are meshed with beam elements. When you scope two shell edges with contact and target elements, we are actually creating a node to surface pair under the hood where the target is a surface along the edge. When the two shell bodies are parallel to each other with coincident edges, we only seem to enforce linear contact robustly. See docs for CONTA175 and CONTA177 for further background. The only workaround for this application is to model the interface with solid bodies.