Rbg Role Change Bug

So here is the issue I am experiencing and I hope some of you fine folks can help me. I am a guardian Druid but sometimes the BG doesn’t call for a tank . So I try to que as Tank and Dps the issue is when I do this it auto selects me into DPS and it won’t let me swap back to Tank and vice versa. I don’t know what to do I disabled all my addons but it has happened three times now. Can anyone give me any advice on this matter. Does it matter that I que into the by as a guardian or balance spec?