Could that be because of the different chipsets between the S6/S7?
The S6 has the Exynos 7420 while the S7 could have either the Snapdragon 820 or the Exynos 8890. So if your S7 has Snapdragon then Unreal will select an Adreno device profile so it may be pulling the settings from different places or pulling in a child device profile that is over-riding something.