I also have this problem, on a Dell Inspiron 15 with I would be interested to know if this is specific to Dell hardware. I am trying the 4.13 experimental to see if that helps.
I guess not just a Dell problem or solved by 4.13.
I read a similar problem on an Antergos forum that placed the blame on Lightdm and suggested a different DM in that case. How feasible is it to switch DMs in KDE Manjaro?