TLDR: Have been playing on my AW3418DW 3440x1440 just fine for a long time, recently upgraded to the fantastic Samsung CRG9 5120x1440 and now fullscreen simply doesn't work.
If I select "5120x1440" and "Fullscreen" the game reloads in 3480x1080, NOT the selected resolution. However if I put it into "Borderless Windowed" I get the correct resolution, but all the performance and Gsync issues