There is an easy work-around, disable the IRIS, the IRIS on the Benq is inactive 95% of the time anyways.
If you absolutely want the IRIS, just go into the service menu and change the settings yourself.
If you keep the PJ in the same general mode (like leaving it in Cinema mode), it will probably not do it. Though I'm not 100%, I believe the problem is from changing settings and the IRIS getting in an invalid state.
Going into the service menu and re-adjusting the IRIS should stop it, choose calibrate aperture and check the min/max aperture settings and aperture strength setting.
My IRIS works fine and has never mal-functioned, and I have 2500 hours on the first 1.03 firmware batch from last year (not the +).
I believe once you reset it in the service menu and fix it, it probably will not do it again.
In the service menu, I would first choose RESET Dynamic Black, then RESET Aperture, then adjust the max aperture to a lower number (82 or below), and then adjust the min aperture higher (6-7 or more). Then choose calibrate aperture, wait a few seconds, then close the service menu.