While it works for most newly opened windows, often times new windows simply pop up on the left-most of my monitors. This is my TV and usually turned off. Typical examples for windows that I find getting "dumped" there: Falkon's download window, dialogs & tool-windows. This is a fairly annoying bug, since I often have to just turn on my TV just to retrieve the missing windows.
please use xprop and xwininfo on affected windows. I bet they specify a user defined position.
I indeed see a user specified location. Guess I need to file a bug against Falkon here. Thanks for helping me debug this!
So (as expected) not a bug in KWin.