Load sharing between 3.3V regulator and LiFePo4 battery
When there is no USB power, reverse current is likely flow from the output to the input of U3 due to the body diode of the internal MOSFET. D6 will block this reverse current at the input of U3, but you are really subjecting U3 to a condition that is not specified in the data sheet. This might cause unpredictable behavior. You could do some testing to see if it causes a problem. But another approach would be to replace U3 with a regulator that blocks reverse current and is designed for this situation.