The spec lacks a line item for leakage current into pin VFB; all that is specified for picking values for R1 and R2 external resistors is to assure that R2>50kOhms.
I am concerned that an existing design that is using R1 and R2 resistor values in the megohms range is suffering from leakage current altering the value of Vout; this might be complicated also with leakage paths present on the circuit board caused by humidity and contaminants. Another possibility is the turn-on transient of the LM27761 having an overshoot if R1 and R2 are extremely high, because the ratio of Vout presented to VFB under transient conditions would be dominated by capacitance of the R1 / R2 parts, not resistance, if the power on time is fast enough.
Is there a leakage spec for VFB?