If you select duplicate, are both screens working correctly?
Even the one that only supports 1366 x 768, or is the resolution set differently on each display?
Yes both screens work correctly.
The resolution is the same on both even though the laptop doesn't support 1920 x 1080 by itself. π€·ββοΈ
But wait! it gets better.
I found a VGA cable and I'm using 1366 x 768 with Intel driver on both displays and without the bug.
Now I wonder if my problem has anything to do with the resolution bug, or maybe the resolution bug isn't about resolution after all but something to do with HDMI output or something else.