Unpredictable behaviour of Screen Settings in Fallout 4
Solution 1:
The issue is that you're using a 21:9 monitor, and Fallout 4 doesn't support that. You'll need to use the mod TruBy9 Ultrawide to allow it to work with this monitor aspect ratio, and manually set the isize W and isize H in (default location)Documents/My Games/Fallout 4/falloutprefs.ini to 3440x1440. Then keep in mind that any time you change your graphics settings you'll need to go back in and change the resolution in the ini.