Quick answer: Unity URP custom renderer feature configured with Inject Point = AfterRenderingSkybox not running? Some pipelines short-circuit when scene has no skybox - use AfterRenderingOpaques instead.
Custom fog pass at AfterSkybox. Works in scenes with skybox; absent in skybox-less scenes.
Use AfterOpaques
Injection point exists in every pipeline; skybox is one of several opaque sources. Reliable across scenes.
Or condition on skybox presence
Custom feature's Setup checks for skybox; chooses injection point dynamically. Complex; thorough.
Verify per scene type
Frame Debugger shows the pass running. Empty = injection point wasn't traversed.
“Injection points are pipeline-dependent. Pipelines vary by scene.”
Choose injection points that exist in every supported pipeline configuration. The least-common-denominator is the safe choice.