Reddit user BraeWhyit claims that SIE sent an email notifying them to that effect. The email basically says that because they bought the game that was selected as a PS Plus monthly free game, the company is refunding the purchase price of the game. As a result, BraeWhyit has gotten a small number of advantages that are not immediately obvious. The first is that the game is left in the library, meaning they are able to play it instead of having to wait until 5 January to claim it.
Secondly, and most interestingly, they get to keep the original license of the game. This means that even if their PS Plus subscription expires, their copy of Maneater will remain playable. Which is usually not the case for PS Plus free games. It’s definitely worth noting that BraeWhyit has a US PS Plus subscription. So it remains to be seen if this refund thing is a US-exclusive thing, or if it applies to other regions as well. (Source: Reddit)