Is firmware for TP-Link MR6400 v1 valid for TP-Link MR6400 v2?

The v2 isn't supported - only the v1 is listed in the Table of Hardware. I regret to inform you that, if you flashed the router with v1 firmware, your device is likely bricked.

https://openwrt.org/toh/start?dataflt[Model*~]=mr6400