In the doc page for state estimation nodes (http://docs.ros.org/kinetic/api/robot_localization/html/state_estimation_nodes.html) it says under ~imuN_remove_gravitational_acceleration that the accelerometer should be providing an absolute orientation measurement to correctly remove the gravitational acceleration from the measurement.

In pull request #247 this was fixed:
6513d7e

Perhaps the documentation should be updated to include a statement explaining this behaviour: If the IMU does not provide an absolute orientation the filter will use the internal state for orientation.