I uploaded a test project.
The issue can be reproduced at Iphone8(IOS 11) and Iphone6s(IOS 10.3.2).
1.Create a Material and set Material Domain to Deferred Decal.
2.Use a Translucent Texture ,and Connect Alpha to Opacity Channel.
3.Create a Decal Actor and use the Deferred Decal Material.
4.Build IOS version and deploy to Iphone8.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-55980 in the post.