Pretty sure TPC has sent you the correct answer here. The 5 year membership expires 3rd January, and immigration made an error stamping you beyond that date. Until a few years ago getting a 6th year out of the 5 year elite was common practice, but more recently immigration had a problem with it - I remember comments on the forum where people were warned they'd be on overstay if they stayed beyond the expiry of their membership. How much this is enforced is anyone's guess, given that immigration officers are clearly still stamping people beyond the 5 year mark.
Edit. Just saw your second email response from TPC. A few years ago they were writing to members toward the end of their membership expiry warning of overstay consequences of staying beyond the 5 years. It seems like their muddled response in their second message to you shows they've since backed down from this position.