This particular decal is animated and has a Translucent decal blend mode
This occurs on both iPhone 8 and 8 Plus (11.0.2)
It does not occur on:
iPhone SE, iOS 11 beta1
iPhone 7 Plus, iOS 10.3.2
1. Package TM-Shadermodels for iOS (QAGame)
2. Install the ipa to an iPhone8 and test it
RESULT: Broken lines appear in the floor decal in the middle of the map
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-50971 in the post.