Most of the native plugin GUIs open a window that is much too big, normally twice the height/width of the real GUI. I suppose this is caused by Qt scaling the complete UI for HiDPI, so it expects that the content of all windows will be scaled as well. This is even the case with Helm, despite its scaling capabilities.
With Calf plugins the problem is a little different, they open a huge window over the whole screen, which they do fill out, but the content is scaled in a weird way. It takes a couple of seconds before the window comes up.
I assume this is a difficult topic, Ardour and QTractor show similar problems (and more crashes for that matter), only Carla gets it right in most cases.