Hello
Starting in 6.1, Fog cannot be configured at runtime by default. This is a tradeoff we've made in order to reduce shader variants and build times.
If you wish to change this behavior, you can set k_DynamicBranchFogKeyword to 1 in the URP config package (this is not documented yet). More info on the config package here: https://docs.unity3d.com/6000.0/Documentation/Manual/urp/URP-Config-Package.html
Let me know if you're still experience problems with this.
- Mikkel