By the way both DDC and dac use I2S clock, I2S is native pathway in every dac...
No. Don’t assume that. Go back to read your Harmony manual again regarding I2s Clock (Sec. 4.2). It clearly says "This feature enables you to select between the local DAC clock or the I2S clock when utilizing the I2S input." The ’local DAC clock’ option is manifest by itself. If you opt for ’I2S clock’, the DAC will be slaved to DDC’s clock and use the I2S clock unless the I2S clock becomes unstable or incompatible.
This is an unique feature offered by Harmony DAC and µDAC.