I had this problem and searched around without any solution, even stumbled across this thread. Thankfully I did figure out a fix and wanted to share since I didn't see it anywhere else.
Look into a program called Borderless Gaming. You can run the game in indowed mode and then use the program to manually set the window size. Once the size is set, restart Sea of Thieves and the aspect ratio should match the size of the window you set
For example, on my ultra wide monitor, the resolution is 3440x1440. I used Borderless Gaming to set the window size to 2560x1440 and it the game displayed perfectly.
You also need to specify the x and y coordinates of where the top left corner of the window should be. Since I wanted my game centered, I set the x to 440 and the y to 0.
All these values will change depending on your screen resolution.