https://bugs.freedesktop.org/show_bug.cgi?id=105046
--- Comment #18 from Lothar Paltins lopa@mailbox.org --- Unfortunately, the issue isn't fixed with kernel 4.18. I've also upgraded my OpenSuse Tumbleweed, but the behavior is the same as before. The screen resolution is correct after switching the monitor off and on about 4-5 times, but it's wrong again after the next power cycle.
My workaround is to execute
xset dpms force off
instead of pressing the power key of the monitor. But sometimes even this doesn't work. It only happens very seldom, but then the screen is only partially blanked and the monitor doesn't go into power save mode. I have to execute the xset command again to make it work.