When the uv value is very close to 0 ( < 0.001 ), the boundary value of the PVRTC texture is sampled as white. please refer to attached images and sample project.
problem goes away when changing the texture format to ASTC
expected : Shading color on IOS device should be same as IOS preview on Editor
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-132321 in the post.