Sorry to hear about this experience. Would you mind changing the default display for your system (macOS / Windows) and then relaunching Illustrator to check if it helps? If the issue persists, kindly share some more details, like the exact version of the OS/Illustrator installed on your computer, the Display Monitors used, and a screen recording of the problem so that we can investigate this further.
This is not realy an issue or bug... I'll explain the context a little more.
I have a plugin written in C++ and I use the CEP framework to create ui panels.
I've created a live effect and I needed a modal dialogue appearing when I edit the effect parameters from the appearence panel.
The problem with CEP panel Modal Dialogues is when they are loaded, they load in the middle of the first screen(single or multiscreen setup). But like other effects this is not what i want. Other effects like for example 'transform' display a modal right above the point where the mouse is clicked.
I allready fixed the issue with some windows functions workarounds as I didn't find any out of the box sollution to this problem.
So can you tell me if there is any way of controling where the CEP model dialogue appears when the extension is loaded?