It basically looks like the setting as "primary" is ignored. When i set a monitor as primary i expect that all new windows are opened on the primary - what else is the setting for? As my third monitor on the left is usally a non active tv and new windows are opened all the time on the left, it is annoying to have to move them at first.
Primary has no meaning for window placement.
*** Bug 398690 has been marked as a duplicate of this bug. ***
But maybe it should? That's what the bug reporter is asking for, after all.
Well, No. This is the most common misunderstanding of X11. Primary has no meaning for window managers. According to xrandr it's only an indicator for the desktop shell where to put panels. As it has no meaning for window managers KWin doesn't read the information at all. Instead KWin has the concept of active screen: windows open on the screen with the active window or mouse pointer depending on setting. And that makes more sense than a randomly configured screen. Unfortunately this hardly ever works as windows position themselves. So it's all mood anyway. What we could do is to explicitly write in the UI that primary has no meaning for window positioning.
And of course we cannot change KWin as it's feature frozen for X11 and on Wayland there's no primary screen.
As it is the most common misunderstand there should be done something about this. I found more posts mentioning and what so ever in forums, reddit and somewhere else, basically everybody mentioning the same thing. A lot of people seem to think "primary" = "First time a window opens is on primary". So basically i have to look into why my windows do not open where my mouse is. It could be the program and it could be kwin, if i understand this correctly. But yes, put something like "Primary (This has no effect on the position of new windows)" or something like this.
If you want to check: xwininfo on the opened window. In most cases it has a user specified position which means by ICCCM that the window manager must honor it. If you as a user hasn't specified that position (e.g. last saved position) it's a bug in the application. If the application uses an application specified position the window manager is allowed to overwrite it and KWin does so.