Tiling a window with mouse at screen edge doesn't always succeed. Sometimes the window jumps to another area and with other size. I don't see any relevant errors at logs when this is happening and I cannot really explain it. I have reproduced this in many distros (Kubuntu, Fedora, Manjaro) with kwidn 5.9.x and 5.10.x. I made a quick video from manjaro with kwin 5.10.5, in order to demonstrate the problem:
*** This bug has been marked as a duplicate of bug 376104 ***