Setting is full screen but game starts in windowed mode every time

Title is pretty self explanatory. This is extra annoying because I have G-Sync and it kicks in and just starts sputtering all over the place since the windowed mode performance during the load screen isn't great. Even though the game is physically in windowed mode, whenever I go into the settings to put it on full screen there is always a random value in the field: sometimes it correcly says that it's in windowed mode, other times it says that it's already in full screen or borderless when it clearly is not. How do I get the game to obey my setting on launch?