Strange, I've never seen this before. I get the same results on google flights (which uses ITA matrix) using 2018/12/12 outbound and 2018/12/19 return in First.
In expert flyer, I found this fare as ANXT04Y5, which has the following in the fare rules:
ACCOMPANIED TRAVEL - MUST BE ACCOMPANIED ON ALL SECTORS IN SAME COMPARTMENT BY AT LEAST 1 PASSENGER.
I wonder if this is some kind of BA sale for two people?