Based on measurements, PG is not functional if Vin is below 1.8V. This PG feature is not documented in the datasheet.
Figure 12 in datasheet shows the PG signal depending on the changes of Vout caused by changes of Vin. But if the pull-up (on PG) is from another independent voltage, PG is active any time when Vin drops below aprox. 1.8V. This behavior has a big impact if PG is used for power sequencing of several power souces.