This anomaly applies to IC Rev. Revision 1, build codes CAAA-C00.
Inserting USB causes VSYS voltage to undershoot and BUCK converter to reset.
USB power source has slow VBUS rise time in the range of 1 V/ms, battery voltage is below 3.1 V, and system load is higher than 370 mA.
BUCK converter resets.
If the system cannot tolerate a reset, avoid operating under the described conditions.