I think it is because Memrise are in the process of replacing their 1 and 2 courses with revised versions. You will only see courses listed in your profile that are available in the main course list (for selecting new courses to learn).
I have something similar with my ‘official’ Spanish Courses. I have been learning Spanish(Spain)1 to 6 for a long time and then found they had revised Spanish(Spain) 1 and Spanish(Spain)2 recently. I have since started the ‘new’ Spanish(Spain)1 course to learn the new content but have yet to start the ‘new’ Spanish(Spain)2. My profile currently lists the ‘new’ Spanish(Spain)1 - plus 3, 4, 5 and 6 - but not my ‘old’ Spanish(Spain)1 or Spanish(Spain)2, because those versions no longer appears on the main course list for selection.
I think you are experiencing something similar. If you go to the course selection list and take a look at the content of the first couple of levels in Mandarin 1, I think you will notice some differences.
Hope this helps!