For those frame rate are cap at certain value (like 240,144...), If disable GSYNC can fix this issue. You can disable GSYNC only for fallout4.exe by using "Nvidia Control Panel > Manage 3D settings > Program Settings", keep the global setting remain enable.
GSYNC issue workaround (1 comment)
You can disable GSYNC only for fallout4.exe by using "Nvidia Control Panel > Manage 3D settings > Program Settings", keep the global setting remain enable.