I'm looking at the Adafruit Feather 32u4 schematic.
It auto-switches between battery and USB power. From looking at the schematic, when the USB power is not connected, the battery power VBAT goes through a MBR120 Shottky diode, then a SPX3819 regulator to power the Atmega32u4.
From looking at the datasheets, the MBR120 has a 0.45V drop, and the SPX3819 has a 0.25V dropout at 50mA. This would be a total voltage drop of 0.7V. Since the regulator outputs 3.3V, the minimum input voltage would have to be 4V.
Since a LiPo battery runs down to about 3.5V when getting low, that would mean that the circuit could not be powered except when the battery was very full.
Am I missing something, or is this how it works? It would seem strange that you would be unable to use most of the battery power.