After I change my username, I find it's back to classic mode, but the parameter in preference is still selected. After I cancel it, save it, and select it again, it works. Maybe it's a bug, please fix it.
Topic on Talk:Structured Discussions
In theory, this bug has been fixed in March. Your Flow board was supposed to be moved when your account has been renamed.
I see you have re-activated Flow on your user talk page on zh.wp, but it is apparently a new page. Didn't you have another one, attached to your previous account?