I have a M6 Device, and also tried comparing Flyme 6 and Flyme 7 by downgrading, listing all bugs and upgrading again and comparing the performance of the phone.

Flyme 6 (Default out of the Box OS)
-some notification bugs (minor, i don't care)
-gallery dates (some videos are dated 1970, but metadata are of correct 2019 date) - I also don't care, i can use Google Photos.
-Front Camera a little too Bright.

Flyme 7 (7.0 to 7.1)
-Phone Heating (heating in just 30 seconds gaming of Flappy Bird,Clash Royale. Flyme 6 heats also, but is less and acceptable compared to my Asus Zenfone Max, Flyme 6 Heat and Asus are Normal)
-Bug in Calculator (my Favorite Phone feature, the percentage and some computations are wrong). I use Google Calculator now. but, yes the Default Flyme 7 Calculator is Buggy and is quite dangerous if you don't know your computations are wrong.
-Opening Apps have delays. slower than Flyme 6.
-Music App Bugs
-One Mind is Broken
-Can't Disable Apps anymore using "QuickShortcutMaker"
-TouchPal is Built-in, Heavy.
The Only good feature is the 3D sound. But, if you are an Audiophile. it doesn't sound right anymore.

My Solution is to Downgrade again to Flyme 6.2 for better performance and Less Heating (Top Part of Phone CPU/GPU Loacation).

Yes, We have done Reseting the Phone After the Update.
Yes, We have Clear All Data after Installation.
It's Just Simply a Flyme 7 Bug and Not designed for our M6. and other Users who experience Heating Issues. Downgrade Now to Flyme 6.2