On windows 7 and 10 (reproduced on my machine and a colleagues), when OS text scaling is higher than 100%, the dropdownLists in scriptUI don't work correctly.
See attached, the horizontal and vertical scaling of the dropdown is smaller than the text it's meant to display, leading to options not being shown.
It looks like the UI is being scaled after the dimensions are calculated, and the dropdown is just not updated at all (vertically or horizontally).
This is a bit of an issue because our design team recently got high DPI monitors and they're having to disable UI scaling in illustrator as a workaround but it makes the entire UI tiny.
I've attached a sample script and how screencaps of what they look on my machine (illustrator CS2019, windows 7)
I can confirm that this is not limited to Windows. This problem exists in MacOS also.