How to prevent different screen sizes from distorting skybox aspect ratio?

I’m working on a mobile game that will primarily be running in a portrait aspect ratio instead of widescreen, but I noticed that when I use this screen size, my skybox its all scrunched up and it’s aspect ratio isn’t preserved. When I switch to a widescreen aspect, the skybox looks how it should.

How do I preserve the aspect ratio of the skybox regardless of the screen aspect ratio?

What unity version?
What skybox is used?
BiRP or URP?
Screenshots would be helpful.

Unity 2022.1.17f1
URP
It happens with any skybox I use, and seems to be related to having the camera setup in orthographic. Here are some screenshots:


So it works without being orthographic?
If the camera is orthographic (so I assume 2D), why do you have a spherical skybox and not just a (parallax) background?

My game is isometric 2.5D, so I use 2D sprites in a 3D space and allow my camera to be rotated within a certain bounds