Hello Carluw,
In order for me to understand this issue better I would like some more information from you.
1.) Are you playing in mobile preview or launching to the IOS device?
2.) What version of the IOS device are you using?
Currently there is an issue with decals not rendering on movable static meshes. On mobile we expect the flag mainly be used to prevent static decals projecting onto players. For rendering reasons on mobile we only support the receives decal flag for dynamic/movable actors.
This issue has been resolved but will not be present in 4.10. We hope to see this in 4.11.