Its not a bug, It does not support widescreen natively. A small tweak in the files works. You need to run the game once. the find a the following folder: Fallout4Prefs.ini in
C:\Users\USERNAME\Documents\my games\Fallout4\Fallout4Prefs.ini
Find the following 4 lines and set up like so:
bBorderless=1
bFull Screen=1
iSize H=1080
iSize W=2560