I have print outs of the ever changing MBIE ‘clarifications’ and "interpretations’ from their website and I cannot locate any reference to an allowance for a drop in visibility. Regardless I believe these interpretations had and have no legal standing, and are not Section 175 Guidance.
60 seconds is a very long time! Visibility is a Building Code requirement so non negotiable. This would be a clear Building Code fail. There were previous discussion on the allowance for a ‘blip’ , but I cannot recall where these initiated. Perhaps one of the draft documents published. From what I recall it was decided to not allow this concession. The drop in visibility discussed was closer to the < 5 second mark. This would allow someone to take a step or two to the side and regain visibility, and would allow for typical scenario where an occupant opens a door and jet of smoke escapes which temporarily reduces visibility.
I generally find that if you use FDS and put in enough measuring devices a detailed analysis will result in visibility being demonstrated.