The water in Hu Xin Pavilion in Gongwangfu can't get in, because the water in Hu Xin Pavilion comes from Yuquan Lake, which is in line with the geomantic view. But water can only enter but not exit, which may mean that water flows into the pavilion in the middle of the lake from the lake, and no longer flows out of the pavilion, but is recycled through the drainage system or natural evaporation in the pavilion. This kind of water circulation system is very common in ancient garden buildings.
Please note that this is only a possible explanation, and the specific water cycle model may require a deeper study and understanding of the history and architectural structure of Gongwangfu to get an accurate answer.