Looking at the code, I have suspicion, that connecting metallic input rule was mistakenly done the other way around. It should sound “Leave metallic and specular unconnected, if not used”.
Because when these two input pins are disconnected, mobile material will use a cheaper shader path. That is the only difference.
When you have specular input disconnected and metalness input connected, there will be no visual difference, but mobile shader will be more expensive.
I suggest you to seek clarification about that from responsible staff and correct me if i’m wrong.
(4.16 MobileBasePassPixelShader.usf, line 42, MaterialShared.cpp, lines 947, 1452 ).