Stereodude,
Given the comments by UncleBuzz, if VUSB was much higher than 5V maybe that can explain the overcharging. Can you check with the 4.3V on the Lipo, what the VUSB is?
In any case with a protected LiPo this should never happen. Plus the protection circuit offers other protections not just overcharging, which is why I would recommend anyone to use a protected cell, more is better when it comes to protection circuits and Lithium cells.
As a "fix" for this edge case (unprotected cell, VUSB>>5V causing VUSB drainage into VBAT):
Inverting the PMOS1 should avoid this problem, or maybe using a schottky instead of the PMOS1 source->drain.
I can do either for you, if you really want to use the unprotected cell, and are willing to do the return and wait a few days.