Start with the Build Profile scene list
Opening a scene by double-clicking it in the Editor is not the same as including it in a build. In Unity 6, the build scene list lives at File > Build Profiles > Platforms > Scene List.
A scene can remain in that list while its checkbox excludes it from the build. Open the failing scene, use Add Open Scenes or drag it from the Project window, then verify that it is checked.
A name lookup can hide a duplicate
SceneManager.LoadScene loads by the name or index recorded in Build Settings. If a name is duplicated, a name-only call can pick the first matching scene in the list.
Short names are fine for a prototype, but old maps and sample scenes make an explicit path such as Scenes/Game/Stage01 easier to audit.
- Choose the affected platform and Build Profile first.
- Confirm the target scene is checked in the Scene List.
- Search the Project window for duplicate scene names.
- If code loads by name, try the intended path or build index and test a new build.
The first scene is a build condition too
The first Scene List entry is the scene that starts the application. The scene that happened to be open in the Editor does not determine the build's startup route.
For a loading screen, LoadSceneAsync can reduce visible pauses. Whichever API you use, check build inclusion before changing several loading variables at once.