I noticed that this happen because we update a rectangular region containing the sensor's origin and left and right sides of sonar cone. So the corners near the origin easily overlap with the area covered by neighbor sensors in a ring. If a neighbor is sensing an obstacle near the origin, and our sensor is clearing a big area, the close obstacle can be removed. This gets solved updating only the mentioned triangle.

I did a simple implementation of this change... If you think it makes sense, I'll clean and make a PR