Time travel is a complicated thing.It has many many different theories.Well in episode 25 or 26 ,Chai Xiaoqi mentioned that planet Cape town denied the fact of parallel universe.So if no parallel universe exist that means Xiaoqi just travelled the same and only one universe from 2021 to 2018.And then they fall in love in 2018.So after the stories of 2018 or season 2 it is very clear that the future is changed.That means their story of 2019 or season 1 will never happen again.So theirs marriage 2021 will never happen.In a word,story of seasons 1 doesn't exist anymore