I'm unable to get play, pause, or TogglePower working on my Harmony 650 remote control, when controlling Flirc v2 USB on NVIDIA Shield TV Pro (2019). The Flirc & Shield are newly purchased. I haven't tested exhaustively yet, but other functions seem to be okay.
(I am able to work around lack of Shield power-up by adding "DirectionDown" & "DirectionUp" commands to the end of the Activity Startup Command list used by my Shield activity. Haven't yet found a way to explicitly power-down the Shield though).