I'm driving a DVI monitor via micro-HDMI -> HDMI -> DVI adapter. I've added "hdmi_blanking=1" to /boot/config.txt but the monitor remains powered up when the screen blanks. With Raspbian/Stretch on older Pis (2B, 3B, 3B+) this worked to put the monitor into low power mode when the Pi blanked the screen. (Of course the older devices used a single HDMI->DVI cable, if that matters.)
Is thre a way to get this working on the Pi4B?