If a loose iris blade (or a rogue ribbon cable) is causing a problem at a wide open aperture, it would be a simple task to look into the lens and see if there is something visible in the light path.

Folks...this is all rudimentary diagnostic troubleshooting. It's part of life, not just photography. And for it to work, you have to be open to the evidence supporting a reality that may be different from your initial presumption.

Fully agree.

Basic 'fault' investigation whilst the 'cause' is unverified should look at all probable reasons and eliminate those where possible. The degree to which this is done is generally determined by the consequences of it occurring again.

One of the significant problems in trying to find out 'why' is obtaining reliable and adequate information from those who were present when it occurred.