Why is the mower still unable to avoid obstacles on a channel after the VisionFence is installed?

The VisionFence has a good two-dimensional visual recognition ability. Through a large amount of data training, it can distinguish the grass from various objects on the lawn, so as to avoid hitting these objects.
Since most channels are not lawns, but stone roads, cement roads, black soil, etc., VisionFence cannot recognize obstacles on this ground. Therefore, on the channels, the VisionFence is turned off, and only the bumper is used for obstacle avoidance.