Had the same issue with Taric a couple of times a few days ago. I just thought I was blind , but maybe not. I know that at least one of the times I tried to ban him he was chosen. Maybe bug with recognizing owned champions?
While I agree that Unyaro makes a good point, I like knowing what they don't have so each of my bans counts
Thinking back, I think that was the week Taric was free... perhaps the ranked system doesn't check free champs?