Autofill bug? Getting autofilled other peoples preferences?

I queued Bot/Mid one of the people in my party queued Mid/Support

I was autofilled into support, while my duo got mid.

How could this possibly work?

The logical, ideal situation is that I would get my second preference, and my duo would get their second preference, instead of giving one their first preference and the other an autofill.

How can I be autofilled a role that somebody queued for?

I like the autofill system, but I don't fully understand it.. It certainly can't be working as intended.