TPS54531: Under no or little load the supply seems unstable

I have used this device in many applications successfully. However, in one design I'm using it as a +3.3V backup supply that has pretty much no load unless a power failure is detected. I'm using an Ideal Diode circuit to "OR" in the backup supply. In about 10-20% of the boards this supply is not stable with no load. The voltage never reaches +3.3V. It seems to ramp up to about +2 volts them shuts down. It's in a continuous loop doing this. What can I do to get around this issue? What is the smallest current load this device requires to be stable?