We need to implement support for reading from the Metal drawable's back-buffer texture for FMetalDynamicRHI::RHIReadSurfaceData FMetalDynamicRHI::RHICopyToResolveTarget in order to make FSlateApplication::TakeScreenshot work in Metal.
Build & run an iOS Metal project.
Attempt to capture a screenshot with Slate.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-18382 in the post.