Why is this MOSFET showing this behaviour?
MOSFETs have an internal diode that conducts in the reverse direction thus you see battery voltage on the diode node: -



Note the diode above. There is usually a diode junction from both source and drain to the substrate in both P and N channel MOSFETs. Here's the P channel MOSFET: -



What usually happens is that the manufacturer connects the "body" terminal to the source terminal so that one of the diodes can never be forward biased and this leaves the other diode able to be forward biased only when drain rises higher than source (P channel MOSFET).