Hi. I actually nuked my Documents/Paradox/CK2 folder completely. Started the game again, switched to borderless, restarted. The game now works properly.
But what's interesting is that I have other Clausewitz engine games which have this exact same issue. I've deleted the folders under Documents/Paradox but that didn't solve anything. I've only recently reinstalled CKII. I think the issue arose after I upgraded to Windows 10 1809. In the past, I've never had any problems with borderless mode.
I've uploaded the requested documents below, but these are after I deleted the CK2 folder and the game is working properly. If you have any idea of what might've been causing this issue, please let me know!