I can confirm this problem too. Stationary PC with Radeon HD6800, Kubuntu 16.04. Monitors do not turn off after certain set time. Only user session locks and goes to login screen.
I've tried Leon's Murer's command "sudo sh -c 'vbetool dpms off; read ans; vbetool dpms on'" and it turns off screens.
I can confirm this problem too. Stationary PC with Radeon HD6800, Kubuntu 16.04. Monitors do not turn off after certain set time. Only user session locks and goes to login screen.
I've tried Leon's Murer's command "sudo sh -c 'vbetool dpms off; read ans; vbetool dpms on'" and it turns off screens.