Higher engine rpm results in higher coolant pressure at the monovalve. Either the diaphragm is allowing coolant flow to the back side (which will prevent the valve from opening) or the spring in the valve is too weak to overcome the higher coolant pressure at high rpm. Or some combination thereof.

Be mindful that the monovalve is closed electromagnetically and spring-loaded to the open position. If you don't believe that the monovalve is the problem, replace it with a piece of copper tubing and you will have more heat than you can stand. I guarantee it.

Your problem isn't electrical. It isn't airflow related. It's the monovalve.