Base game uses
circuit_wire_connection_points = circuit_connector_definitions["belt"].points,
circuit_connector_sprites = circuit_connector_definitions["belt"].sprites,
which is defined in prototypes.demo-circuit-connector-sprites

Mod uses
circuit_connector_sprites = transport_belt_circuit_connector_sprites,
circuit_wire_connection_point = transport_belt_circuit_wire_connection_point,
which is not defined anywhere.

This result in engine can't find a defined connection point, defaulting to the center of entity. Simply switch to the vanilla should fix the problem.