Does forcing a dpms cycle also work?  E.g.,
sleep 5; xset dpms force off

Does turning the monitor off work with other drivers or OSes?  DP requires link
training to bring the link up between the monitor and the GPU.  I suspect when
you physically turn off/on the monitor, it does not generate a hotplug signal
so the driver had no way of knowing that the link needs to be re-established.