There’s a theory I like that accounts for this. According to the theory, FL’s daughter and husband are translucent because Ep 22 is FL’s dream. But it’s not just a normal dream, it’s a prophetic dream much like the one she has at the start of Ep 1. So even though the happy events during New Year’s Eve are just FL’s fantasy, they will come true.
At the end of Ep 21, Qianyue’s daughter is old enough to speak, suggesting that three years have passed. The bell rings and Qianyue turns around and smiles with tears in her eyes, suggesting that Changfeng did end up returning. The appearance of the translucent Changfeng and daughter occurs at the start of Ep 22, this marks the start of the dream. Changfeng returns at the end of Ep 21, before the start of the dream, which suggests that Changfeng’s return is true. So in the last scene she wakes up and smiles because all is or will be well.
At the end of Ep 21, Qianyue’s daughter is old enough to speak, suggesting that three years have passed. The bell rings and Qianyue turns around and smiles with tears in her eyes, suggesting that Changfeng did end up returning. The appearance of the translucent Changfeng and daughter occurs at the start of Ep 22, this marks the start of the dream. Changfeng returns at the end of Ep 21, before the start of the dream, which suggests that Changfeng’s return is true. So in the last scene she wakes up and smiles because all is or will be well.