Quick answer: Unity build stripping the procedural skybox's exposure keyword, causing default sky? Add the keyword to the always-included variants or assign a skybox material explicitly.
Custom procedural skybox with exposure animation; build renders default blue sky.
Add to variant collection
Author a ShaderVariantCollection with the procedural skybox variants. Ship via Resources.
Or assign explicit skybox
Lighting tab > Skybox Material. Hard reference; cooker preserves.
Verify build report
Shader variants listed; procedural skybox entries present.
“Shader stripping removes unreachable. Procedural skyboxes need hard references.”
If you author a custom skybox, the variant collection is the standard preservation pattern.