You're expected to be at least competent in every role, that's part of why autofill exists. It's a small remnant of the days before we had role selection, when you would get to pick your role based(Mostly) on pick order. It was a very common statement to hear "You should know how to play every role before you queue for ranked, it's your own fault if you get stuck with one you don't know/play and lose because of it.". I kind of agree with how it works to be honest, part of being x rank is knowing all of the roles well enough to hold x rank.
It can suck when it does happen, but it's motivation to go and practice whatever role you got autofilled to and felt inadequate. At least these days if you warn your team that you're autofilled you can often get a swap(If you ask for it), or at least some leeway from your team if you end up sucking. Previously people would just flame non stop for not knowing whatever role you got stuck with, it's certainly improved quite a bit from those days.