MCP73844 Li-Ion charger
Best guess is that the misleading / verging on wrong example circuit diagrams in the data sheet have lead you to reverse the FET drain &amp; source connections.

The FET source should connect to Vin, not to battery, despite the FET-symbol on their diagrams suggesting otherwise. This is a P Channel FET (both their one and your replacement.) The symbol used incorrectly implies an N channel FET with drain to input. 

For the FET to act as a FET the body diode must be as shown on their diagrams  but to do this the drain must be to battery when a P channel FET is used. If d &amp; s were reversed the FET could not be controlled as the d-s diode always conducts.   

As Vin is 10V and Vbat max = 8.4V &amp; less if depleted then charge I may lift Vbat due to IR drop &amp; ...