"mix position" is called Front of house, or FOH. The "Haze" you are referring to is from a fogger, this isn't just water mist though, it's a chemical mix that is slightly opaque to create that "beam" effect with the lights, so it's not as simple as filtering because the fog itself is not transparent.
Yeah, I've been an audio engineer for over 20 years. I've mixed FOH and monitors regularly since I was a teenager. I also have quite a few years experience in event production and currently serve at my church as the Technical Director overseeing a team who handle all aspects of technical production. I only recently (last year) got interested in photography. I used "Mix position" as a generic term since I doubt as many photographers would understand the FOH acronym.
This "haze" was actually haze. Fog uses a different chemical solution and is a much heavier, short term effect. Haze (oil based solution) is a different chemical mix that is designed to last a lot longer and "hang" in the air to help produce those lighting beam effects that everyone likes to do now. I don't want to bore anyone with the technical details though.
I don't even know how many concerts and events I've photographed in the past year, but this was actually the first where haze was being used (and quite heavily). I'm a newbie photographer, and as such, have never run into this issue before, so I surmised that the haze may have caused the focus issues and thought I'd ask the professionals here before I write it off to cheap equipment.
I suppose it is a little reassuring that you have experienced similar issues. At least that points to the haze/smoke/fog as being at least partly responsible.