No worries! In that case, make sure you set the correct resolution in both Unity's Player > Resolution and Presentation settings and itch.io's settings - that worked for me to keep my game in portrait mode.
If you don't mind, I'm actually facing a similar issue here where my game gets a bit cut off at the top and bottom during full screen while playing on PC. It works completely fine on mobile though.
I'm wondering if this is happening to you as well?