The same happened to us when we tried to help an FC member with A1S. I accidently clicked us into A1S as I thought it would count our progress from the lowest person, but for some stupid reason the game decided that we could take a person that had yet to clear even the first floor into the third one. After that he was locked into that floor for the rest of the week, which sucked.

I don't know why it works like this. Theoretically you could always have some team taking you into A4S weekly until you clear it, without ever touching the precious floors. I think this is (yet another) hindsighy in SE's programming that they can't be bothered to change.