Negative voltage from buck regulator
D18 and D16 will rectify the AC voltage at pin 8 of U14. It is similar to a voltage doubler rectifier.

This can work usefully however it relies upon U14 continually producing a square wave at that pin.  It may not do so if the load on the main output (7.5V) is at a low current level.

Many switching regulators go into various low power nodes when the load is reduced - pulse skipping, PFM (pulse frequency Modulation) or similar.  These will result in the signal to D18 not being continuous and could give the symptoms you see.

Make sure you have a load on the main output - use a resistor to simulate the final product load if necessary.