Update: downgraded to v5.1 from this link: viewtopic.php?f=96&t=16680
On the positive side, I'm back to a stable phone which is able to charge the battery at full speed (current = 1.26A while powered on).
On the negative side, the 5.1>6.0>5.1 experience left a bug: at every boot the system language is changed to the one of the SECOND SIM card and the keyboard is set to AOSP keyboard in that language, which gets automatically re-enabled. I guess that is a problem with the radio which probably did not get downgraded. Do you have a solution? Can you please release a ROM without that bug?
Some notes about the process:
Downgrade through SD card (About Phone > Wireless Update > Local updates) did not work:at reboot the phone refused to downgrade. So I had to do it through the computer: find a SP flash tool and drivers from late 2015 (because older drivers and tools do not recognize MT67xx). I flashed all the partitions in the zip and it worked.
A little rant: this is my everyday phone; I can't afford two days downtime like I had. If this one gives me hassle then I just buy another brand. If you can't afford proper QA testing, then please leak the full ROM sources so that the open source community will do QA for you. Doing so would even increase your sales.
here a sincere apology for the problem.
we wil release an advanced verison ROM to improve the performance.
but new rom release need some time, please wait patiently. we will release it
as soon as possible when it finished.