UPDATE: This is primarily due to the fact that it runs as Fullyrough on Mobile as long as a Constant 1 value was feed. Also on Mobile the FullyRough shading looks like simply remove all Highlights/Reflection instead of respecting the rule of Energy Conservation. Here has a Bug related
But still there's quite a difference between the two platform, I will rise another ticket for the issue there.
Unzip the folder attached into Content folder of 4.26 engine and open the map inside.
Switch between Mobile/ES3.1 and PC/SM5, see the difference.
The reflection looks much darker on mobile preview vs pc
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-106218 in the post.