https://bugs.freedesktop.org/show_bug.cgi?id=106363
--- Comment #6 from Gregor Münch gr.muench@gmail.com --- Thanks for your explanations Alex.
Maybe useful for other: It turned out that Steam client has probably a bug preventing screensaver getting active.
https://github.com/ValveSoftware/steam-for-linux/issues/5532
One more problem seems to be that Deepin or whatever still cant run the screensaver even after Steam was closed.
https://github.com/linuxdeepin/developer-center/issues/567
Im sorry again for occupying this bug report.