I’m also seeing this issue on release 1909 when more than one monitor is active. Input from the physical mouse and keyboard are properly blocked, but none of the displays are blanked.
I found thisĀ https://www.nomachine.com/TR10Q09447, but I don’t totally follow its wording. None of my three displays are blanked; I’m not sure if the TR is implying that it should function properly on the primary display.