Skybox material back to default after light build

Hi guys,

I am trying to change the default skybox material to a custom one that I have created(a space one). I have changed the static mesh of the sky blueprint at the “skybox” one. I have also created a skybox material from which I change the texture every time and everything works fine in another project but in this one I can’t change the material of the sky blueprint unless I open it but even after this after I restart the project or build the lighting, the sky material is going back to the default one. Although, this problem occurs only in this project. What is wrong?