After the recent MANDATORY firmware updates rolled out by Xiaomi, flashing custom firmwares with downG and the ScooterHacking Utility android apps fails:
the main issue seems to be the newer dashboards coming with BLE157 which interferes with the flashing of nonapproved firmwares.

I initially thought that the ESC micro had something done to it to make it more secure but just swapping over the dashboard has worked for me.

I would expect this method to become redundant soon :)