This could even be seen as a security issue - the display isn't hidden at all using the screen lock, the only thing it does is making it impossible to interact with it. So it is visible what the user was doing when leaving the computer for some time.
Upstream bug: https://github.com/mate-desktop/mate-screensaver/issues/103 It appears to be fixed with this patch: https://github.com/mate-desktop/mate-screensaver/pull/110 It would be nice if this could go into stable stretch. -- Ayke
