All windows on my systems open on the top/left position (x:0 y:0). I tested this behavior on any systems (tested only on Arch Linux) -> same behavior.
How to reproduce:
1. Open Dophin -> It opens on position 0:0
2. Move it to another position & close it
3. Reopen Dolphin: it opens not on last position. it opens on position 0:0
BTW: kwin remember last window size, but not the position
*** This bug has been marked as a duplicate of bug 15329 ***