The Galaxy S6 has a Mali-T760 GPU I believe. Updating to Marshmallow would include a new driver which is likely why the rendering changed. Which version of UE4 are you using? 4.11 has a number of fixes for mobile rendering. I would be best to create an AnswerHub with the model and logs from the device (adb logcat -s UE4, or use monitor and add a UE4 tag).
This occurred whilst using 4.11. I updated to 4.11.1 and it’s still happening I’m afraid. My other mobile game seems fine though (apart from a small screen banding issue when using bloom which wasn’t there before Marshmallow)
I just read that 6.0.1 is coming soon to my T-Mobile S6 and now I wonder if it’s worth updating (it seems rendering and performance issues plague S6 + Marshmallow in regular and VR apps/games using any game engine)