With nouveau drivers (current versions: kernel 3.15.9,
xf86-video-nouveau-1.0.10, xorg-server-1.15.0; however the issue exists since
several years)
"xset dpms force off" (or standby/suspend) does not work here as it should:

In all three cases, the monitor switches off for about 20 seconds but then
switches on again.

The issue is completely reproducible and does not occur with the proprietary
nvidia-drivers.

I am rather sure that nothing (except perhaps nouveau drivers themselves)
causes any activity which might cancel the effect of "xset dpms force off":
This is already proved by the fact that it works with the proprietary
nvidia-drivers, but I also checked many times that no suspectible task is
running (and besides systemd the bug has occured also with openrc and with a
minimal number of started services).

As mentioned, the problem exists since several years, so I do not think that
any of the recent changes referred to in bug 76483 is related to this.