Ok, after TeamSpeed posted that his Σ8-16mm works on every camera under the sun, some of them newer than my 80D, I decided to start over with my 12-24 on the 80D. I got home this morning, and ran the focus from MFD to infinity a few dozen times, cleaned all the contacts, and re-flashed the 80D firmware. I'm not sure what exactly fixed it, but it is now working. My 80D can lock focus at any focal length, wide open. It is a bit slow, but that is the lens. I only tested out to about 5'; at that distance and f/8, the entire world is in focus. More importantly, it works just as well at MFD and wide open. It even works in light low enough to kick on the annoying 80D flash assist strobe.
Thanks all. Now I start my hunt for a nice Sigma 8-16mm. Anyone need a Sigma 12-24 II? I am still debating whether or not to sell the 6D. My fast(er) primes really shine on the 6D.