Just to completely rule out the bad SPD idea, can you please dump the SPD content when you boot with the production BIOS? i2cdump output (ASCII) or raw output from sysfs would be great. Then we can plug it into bc and confirm that the checksum is correct. This would also be a good way to confirm that the SPD addresses are correct in Coreboot, and rule out the possibility that ASRock changed SPD addressing between board revisions...
Attaching /sys/bus/i2c/drivers/eeprom/0-0050/eeprom as hex file + i2cdump output