As for why Dayong and Qingyi can see the light rain? This is because the "rule" of piano music transcending time and space is "Every time you come from the past to the future, the first person you see can see you." (Note that it is "every time")
Moreover, every time Xiaoyu comes to see Xianglun, she will go back to the past on the same day (does she always go home for dinner and sleep? When Xiao Yuqing came to the future again, the "rules" came again. Xiao Yuqing had to close his eyes and come to the classroom in a few steps to make sure that he could see Xiang Lun at first sight and let Xiang Lun see her.
Later, Xiaoyu closed his eyes and came to the classroom. At first, he didn't see Xiang Lun, but he saw Qingyi, so Qingyi could see the light rain, so they spoke normally in the piano competition.
As for Dayong, he is in front of the future piano room, and Xiaoyu bounces from the past to the future. When he opened the door and left the piano room, he saw Dayong at first sight, so Dayong also saw the light rain.