Can spread spectrum clock be applied to SYNC pin?
PLL_Unlock status can be seen on the LTpower Play and I like to confirm spread spectrum clock can be used as external clock source.
From the LTM4676A operation point of view, it can track to 1-2kHz rate change. In your application, what’s the switching frequency and the modulation rate? If the modulation rate is higher than 1-2 kHz so that the LTM4676A couldn’t track it. That's why you see the PLL fault.