@Elaine I don't know which one of them is marshmallow feature or not but these are the problems I am facing and some of them were present in lollipop too.
List of problems in Alcatel Flash 2:
1) RAM consumption is increased.
2) Battery backup is reduced.
3) After playing a song (earphone plugged in) there was no sound unless I press the volume button. Music player in which I have faced the problem - Google Play Music and Poweramp.
4) Rear camera- Can't produce the actual colors since from the first day. After upgrading to marshmallow the quality of photos is even more degraded.
5) Front camera- In lollipop the performance was ok but after upgrading to marshmallow, I stopped using it.
6) Set the brightness level to lowest you will notice that the screen is flickering. But it requires a white background then only you can see it clearly.
7) Earphone sound is reduced.
8) Notification LED blinks like a speed of a bullet train.
This phone was lauched for photography, I am not a professional photographer but still I can tell you that there are many smartphones with better camera performance in the same price segment. And camera performance was a problem from the beginning but still I can't see any improvements and it's becoming worst day by day.
Personally I prefer to take photos with Xiaomi 8 MP camera rather than my 13 MP camera.
Overall just wanted to say that I am totally unsatisfied with this smartphone.
This is why i wanted to downgrade to lollipop. It has less problems in comparison to marshmallow.