I've seen a few of these "v255.255.255.254" version numbers, usually in FPGA module. Upgrading the FW leaves these modules intact, so at first I though this is a way DJI uses to avoid upgrading the module (ie. if a component needs a work-around in firmware, the current FW version is set so high to always report that it's newer).
But now I'm not that sure anymore. This may be just a common glitch which happens in the FPGA chip.
What's your downgrade log?