Why are enemy Flipbooks not loading on mobile?

In your material editor, hook up a [feature level switch][1] node to your masked unlit sprite.

Feature Level Switches:

Default: The default Feature Level.

ES2: Feature Level defined by the core capabilities of OpenGL ES2.

ES3.1: Feature Level defined by the capabilities of Metal-level devices.

SM4: Feature Level defined by the core capabilities of DX10 Shader Model 4.

SM5: Feature Level defined by the core capabilities of DX11 Shader Model 5.

Utility Material Expressions in Unreal Engine | Unreal Engine 5.1 Documentation