Scenes are removed from build settings

I have a project, where I at the moment have two scenes. A preloader scene, and a game scene that is launced from the preloader scene.

Something strange is happening, every time I open a scene, the other scene is removed from the build settings. So when I've been working in the game scene, and then open the preloader scene to start the game, the game scene is removed from the build settings and I get the error about the scene not being in the build settings.

I've tested this in both version 2017.3.1f1 and 2018.1.0f2, and the problem exists in both versions.

Anyone have any idea what is happening?

 

Problem solved.

It was one of the unity packages I'm using that for some reason did this. Removed the editor script responsible and everything is back to normal.