About autofill

So I play support. Naturally autofill is pretty much activated every game to get me the role I want. Mid is second choice. And now it happened several times that I got bottom instead. Which inevitably means I am not support, but adc. So much for protection. Why is that happening?? If I planned to play adc I'd pick adc, I mean after all I'm supposed to be GUARANTEED to get the role I want. Bottom does not equal support. That's a really flawed system.