How it's possible thet it is accidental coincidence not related to the Android version upgrade. A lot of people write about slow charging after upgrade. Even I have this problem. This is why I'm still on Marshmallow.
I think that accidental coincidence is that on Nougat this phone charge 2h.
I'll try to opgrade to this version with cache cleaning on weekend and check if this helps.
It's surely related to Android 7. I have this problem too and I wrote about it. My battery is charging whole night and when I look at my phone at the morning, it's charged to only 70/80 percent, depending on how much it was before charging. Today when I was using only messenger battery was being DISCHARGED WHILE PHONE WAS PLUGGED!
Facebook stopped working notification really annoys me. And of course Facebook app is still working but the notification appears when it wants. Clearing cache, factory resets, reinstalling update don't work, so it's firmware fault that began with Android 7 update. Wish there was a custom ROM for Umi Max
I bought the phone after I saw it had regular updates... Unfortunately I think the devs are busy with yet another new phone... Umi dont get your work half done, please! The hardware is great, make the software work as it should...