Have a centered panel, make it not full-screen width and then maximize it - the panel jumps to the screen on the left. This can be reproduced only once after plasmashell startup, therefore I think it may be related to bug 339554 and possibly the cause can be the reason why bug 339602 happens. This is still present after the last batch of the panel patches.
Is an upstream issue, with a proposed patch here: https://codereview.qt-project.org/#/c/97050/
Marco's patch is in. Also 56b14c7b17ce3e32c5530b6f0f5b09ce1361d744 and eefca4383e92e7b15d6cdda6882376d02605273f should be fixing this problem.