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LTC4353 secondary input voltage drop

I'm experiencing a strange behaviour on a board where I'm using LTC4353 as a "12V Main and Auxiliary Supply Diode-OR" . The circuit I'm working on is depicted below (it's pretty the same circuit found on page 11 of the original datasheet)

Vref value is provided trough a voltage divider:

Testing it I found that it correctly switch between power sources around 9V but while the main source (+12V) is correctly feed into +12P output the secondary (+BATT) is dropping 0.6V..... I don't think this's a correct behaviour but I cannot understand what's the origin ... probably Q1 is not saturating and this lead to some voltage drop but why?

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