You might be assuming zero wind. Stationary hovering with no wind fits what you've said. However, when you add in wind, the downwash from the upper rotors will not be perfectly vertical and will be influenced by the wind direction and speed. It's easy to envision the downwash moving downward at an angle that would cause it to intersect with one of the lower rotors. Adding or subtracting a few knots or a few degrees could correct or exacerbate the effect. Exactly what wind speed would cause a loss of lower rotor effectiveness would be an interesting vector to work out. Perhaps that would be a design limitation that could be demonstrated and placarded.

I envision it like doing a 360 hovering turn with a good wind in a conventional helicopter. As some point, you hit a spot where the tail rotor loses effectiveness as the wind affects its angle of attack. Mentally flip it 90 degrees and imagine the wind being the downwash from an upper rotor and you'll get what I'm thinking about.

Good luck!

Paul A.