Steps for reproduce: 1. Start Dolphin and unmaximize it. 2. Remember Dolphin window position and close it. 3. Start Dolphin again. Actual result: Dolphin window started in top left corner. Expected result: Dolphin window started in previous position.
There is no support for remembering the position of a window. I'm sorry this is out of scope both on X11 and Wayland. Windows in KWin are placed based on the placement policy, but not on the last position.