Aspect ratio 21: 9 (Little Nightmares)

Hello everyone, please tell me what to do - in the game I put up my native 3440x1440, the interface and the game menu on the whole screen in 21: 9 format, but as you start playing, the game takes the 16: 9 format and naturally black bars appear, but the interface is also 21 : 9, what to do in this case, I want to play in full immersion.