A SOLUTION HAS FINALLY BEEN FOUND:
Thanks to fatcrobe from Reddit this issue is finally resolved for me and here is the solution:
The Windows UAC (User Account Control) is the source of the problem. If you have this disabled for whatever personal reason, enable it and or alternatively download and run this troubleshooter to fix it for you.
I said I was going to get angry so here is my rant for the millionth time. Sorry about the length but I want to speak my mind fully this time as this has been completely killing my joy of playing for the longest time.
You have seriously had all the time in the world to do something about it but you decide to launch the Beta anyway knowing very well that this issue exists for a good while now and that it was guaranteed to affect a number of new people who Pre-Ordered or are playing for the first time. How much do you actually care for everybody to have an equally good experience? I understand if you cant please everyone with the amount of content in the Beta, but you could at least make sure the game is accessible and functions properly for everyone, especially when you've had all this time to do it. This level of carelessness is beyond frustrating.
These are from today, I would link to more older threads in here but a lot of the older Alpha threads have been deleted. I'm sure there will be popping up more threads and reports further into the Closed Beta... Heck, why should it matter anyway? you clearly know about the issue at hand:
https://www.reddit.com/r/Seaofthieves/comments/7spp41/how_hard_is_it_to_fix_this_mousemultiscreen_bug/
https://www.reddit.com/r/Seaofthieves/comments/7spvd1/windows_10_issue_full_screen_mouse_capture/
https://www.reddit.com/r/Seaofthieves/comments/7sphre/game_window_broken_with_multiple_monitors/
Let me just remind you the game was perfectly fine in the Alpha until at some point there was an update released that completely broke how the forced Borderless-Window "Fullscreen" is recognized by the game when you have multiple monitors for some users, since then people affected have been reporting on it but instead of fixing their own errors Rare have clearly been completely neglecting the issue for some reason, maybe because not enough people are experiencing it for them to care enough to bother?
As they explain in this blog you would think this different approach and choice of development would prevent issues like this still existing after months of reporting and that Rare's goal was supposed to have a good level of quality at all times? Being forced to disconnect your other monitors just to be able to play the game without constant frustration and then yet still having issues is NOT a delivery on a good quality experience. Completely unacceptable at this point.
Get your act together Rare!
/rant