LT3791 Buck mode question

Hello

      Customer has and issue about LT3791EFE-1 and that is when the input voltage > output voltage, the buck-boost converter should be worked at buck mode, and the M1 & M2 should be work as buck converter, and M4 should be 100% on and M3 should be OFF, however when they measured the waveform at boost side of inductor, it is not a flat waveform like Vout, it is also a switching waveform, that violate customer understanding about H bridge buck-boost converter. 

      Why we see switching waveform, on boost side of inductor?  Is it normal? Thank you. 

Patrick