Hello.
Unfortunately the issue is that URP SG Unlit target is not, and never was, designed to be used as a skybox material. It only ever happened to work by accident, but URP does not officially support it.
Instead, you can try the Built-In Unlit target instead with
- Depth Write: Force Disabled
- Depth Test: L Equal
but note that this is not officially supported either.
Sorry for the inconvenience.