I have the Dungeons Launcher version instead of the Windows Store version, but it works if you rename the EXE (found in C:\Users\{UserName}\AppData\Local\Mojang\products\dungeons\dungeons\Dungeons\Binaries\Win64\) from Dungeons-Win64-Shipping.exe to Dungeons.exe. I expect I'll have to do that every time there's an update, but it's better than playing the game in 16:9. Thanks!