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OK, after some study and experiments, I guess I find the reason. The obstacle marked/cleared by ultrasound is performed by range_sensor_layer::RangeSensorLayer while the obstacle cleared by Laser Scan is implemented by costmap2d::ObstacleLayer. It is simply that they are at different layers so that one cannot be cleared by the other.

I provide my findings here so other people who might face similar problem can get some insight from this post. The concept of layers used by costmap is the key to this problem.

Best regards, Gerry