Howdy. New here as well. Yes, I believe you must be in MF mode. If you are not, when you release Fn1 and push down on the shutter button you are engaging the autofocus and throwing away your MF effort.Must it only be in "MF" shooting mode?
I have redefined the Red Dot video button to be used to turn manual focus on and off. I have had Fn2 defined as the Magnify options to assist with manual focus, but with this hint, I will redefine it to be AEL/AFL to give me push button focusing to start and then fine tune with Fn1. I will also set my autofocus area to be in the center of the field.