i had a similar issue, connecting RPI with HDMI vs. VGA had different behavior.
To get my PIR working the way i want it, i had to use a HDMI->VGA Adapter and connected the display by VGA. Dont worry, VGA also supports FullHD Resolution.
See alse: https://forum.magicmirror.builders/topic/6410/which-monitor-supports-standby-sleep-poweroff-wakeup-via-vga-hdmi-commands