Thanks for the information.
This si working as intended. Once the player enters the final six months of his contract, the player is able to negotiate with any AI club, regardless of the clause in the players current contract.
If the user had activated this option before, or during Mata's negotiation with the AI club then his contract with Galatasaray would have been extended and the deal with the AI club would have been cancelled.
However once the deal with the AI club has been signed the optional extension clause with Galatasaray is void.
Cheers,
Ben