I have an Alienware Aurora R9 with Intel i7-9700, 32G of RAM, NVIDIA GeForce RTX 2070 SUPER graphics card, and 2 monitors (1920x1080) and a Huion pen display (2560x1440). The pen display is new. I have both Illustrator v25.2.1 and v24.3 installed. I use no 3rd party plugins.
When I open Illustrator, the cursor immediately jumps to the left corner and the GUI resizes large enough to fill all three monitors--which means the bottom 3/4 of the program window lies below the visible area. The cursor continually jumps about until the window is resized this large. I did the usual; updated drivers (which I had to do because of problems in Photoshop and Premiere Pro), turned off GPU, unclicked the "use Wacom" box because I use Huion. Nothing worked. I finally made the widow smaller than the pen display (about 1/2 the size) and the resizing issue did not return. If I carefully resize the window manually, I can cover most of the display, but there comes a point where it automatically becomes full-screen, then I have issues.
No matter the size, the cursor still jumps about when I attempt to leave the program while it is open on the pen display (like moving the cursor to another monitor to answer an email). If I manage to force the cursor onto another monitor, within a second or two, it jumps right back to Illustrator. I have to either minimize or completely close the program to use any other program. If I move the Illustrator to one of the other monitors, the cursor moves on and off the Illustrator window just fine, but the resizing issue remains--and if I attempt to move the window back to the pen display, well, I usually can't. The cursor jumps about so badly I can't get it to stay in place long enough to click on the bar to move the window. No other program on my computer, including Photoshop, Premiere Pro and After Effects from CC, has similar resizing and cursor issues (though the Adobe products still have issues all their own. Every single one I've installed/upgraded in the last couple of months has had issues to the point I've either had to install a previous version to get the program to work properly or use another program outside Adobe CC to get work done).
I thought, perhaps, the screen resolution might be the culprit, but my laptop also has a Huion pen display attached to it and the screens are the same resolution. (that 1920x1080 Huion pen display was originally attached to my desktop, and it also experienced difficulties after the v25 upgrade). I read a few posts from 2017 and 2018 about resizing issues, but the solutions listed don't work for me.