My macOS SwiftUI project has minimal target 10.15 (Catalina). When I run a build made in Xcode 13.1 on Catalina there are multiple ui issues (views has wrong dimensions, list items has wrong offsets), however when I run same project in Xcode 12.4 on the Catalina laptop issues are gone. Since I cannot replicate the issues in Xcode 12.4 I have no ideas how to tweak my Xcode 13.1 build to make it work correct on Catalina devices. I cannot use Xcode command line tools 12.4 in Xcode 13.1 and vice versa 🤷🏻♂️
SwiftUI views issues in macOS Catalina
I also ran into the same problem so I gave up and just move my minimum target to 11 (Big Sur). I don't have the issues anymore but that doesn't fix your problem with trying to deploy to Catalina. It seems like Xcode 13 is focusing on newer OS versions. Even SwiftUI is deprecating a lot of functions that support 10.15. In fact, all the new functions are only for macOS 12 like .overlay and material background.
Such a tough choice regarding having to support older OS since Apple breaks the backward compatibility so quickly so people have to move to the new version.