I queued mid/fill twice in a row and got support both times

Don't queue for fill if you don't want support.

Autoprotect = you get GUARANTEED either primary or secondary. secondary = ANY = Support

But, I never get support when I put it as my secondary.

I queued support/jungle for all but 1 ranked game. Always got support.

If I wind up in jungle, my team is going to be more screwed than normal.

Fill basically means that you will take any role if your primary role is not available. Support is the least desired role so you have a high chance of getting support if your secondary is fill.

autofilled is getting filled when you dont queue for fill, but you should be autofill protected for when you remove fill as a choice

Fill/Autofill is just a fancy name for support

As already pointed out, it's because "Fill" means "Support". To me it only happened once that "Fill" got me the Jungle, but by then I was so used to "FIll=Support" that I almost ended up Sona jungling without Smite. ("Almost" because someone dodged while I was trying to get an actual jungler and the correct spells/runes/masteries in the 5 seconds left).

glad you got what you wanted