Hey ,
Could you please provide additional information about your issue?
- Which version of the engine are you on? Help > About Unreal Editor
- Are you on Source or Binary?
- Did you create the materials yourself?
- Are you able to reproduce this in a new project?
- If so, what are the steps to do so?
[Unreal Engine 4 Documentation: Mobile Previewer][1]
Using the Mobile Previewer in Unreal Engine | Unreal Engine 5.1 Documentation