Yes it was ECC -- just not only what was written by the vendor but also all writes done using the old driver...
And yes, the installer only writes back the EEPROM data and MAC addresses inside mtd2, writing the whole partition didn't seem to be needed (and would also be slightly more tricky as there may be bad blocks between EEPROM data in the beginning and MAC addresses in the middle, so that the offset of the MACs would change ...).
As @981213 explained well in https://github.com/openwrt/openwrt/pull/4179 , the root-cause is a mix of API misuse by mt76 and mtdblock (not handling EUCLEAN) and apparently broken ECC data for things which had been written using the old driver.