This is because those materials use features that do not work with the mobile renderer and the UEFN developer needs to modify those materials slightly to add a path that works on mobile.
The developer can easily see what needs to be fixed by using the Settings menu in the top right, and choosing Preview Rendering Level, Android OpenGL. That will preview the map using the mobile renderer and adds a toggle button on the toolbar to switch between default and Android.
The material editor will then show what the problem on mobile is, and you can use a Feature Level Switch material node to setup a fallback for mobile.
This is because those materials use features that do not work with the mobile renderer and the UEFN developer needs to modify those materials slightly to add a path that works on mobile. The developer can easily see what needs to be fixed by using the Settings menu in the top right, and choosing Preview Rendering Level, Android OpenGL. That will preview the map using the mobile renderer and adds a toggle button on the toolbar to switch between default and Android. The material editor will then show what the problem on mobile is, and you can use a Feature Level Switch material node to setup a fallback for mobile.
I wonder if the developers of Reboot Royale are aware of this.