I believe this post should be in the iOS bug section and not the Chinese language section.
I just finished the official Mandarin Chinese 1 which ends on chapter 26. When I on the iOS app clicks on [ Continue ] for Mandarin Chinese 2 it then jumps to chapter 26 there.
Finishing that chapter (26) will make it continue on chapter 27 and ignoring 1-25. When I enter Mandarin Chinese 2 in the web browser it just normally starts at chapter 1.