The World Cup winner's current deal runs until summer 2028, with the midfielder having two years remaining when he spoke publicly in September.
Mac Allister stated that he was "very, very sad" after not being offered a new deal and cited Dominik Szoboszlai and Ryan Gravenberch's new deals as examples of him being let down by the Reds.
“The fans don't need to worry because I will give my 100 per cent until the last day I'm here," he said earlier this season as supporters call for him to be handed a new deal. "I love being here, and this is something special.”
Speaking to Betfred, Johnson admitted that he is dumbfounded as to why Mac Allister is still left in the dark, despite his fine form.
"I am surprised. He's always solid, he's never terrible and he's a great player in Liverpool's team. I really like him as a player and Alexis has openly said that he wants a new contract because a couple of the other players have had their contracts renewed and he hasn't. He's clearly as shocked as us.
"If you're good enough to start every week, then he's surely somebody that you want to keep. He does everything well and other top teams would bite your arm off for him, so I'm surprised it's not been renewed yet.
"Maybe there's internal information that we're not aware about, but as a player and how much he helps the team, then I think they need to keep him."
Mac Allister has publicly reaffirmed his commitment to Liverpool, but reports state that European rivals Barcelona are interested in the playmaker who could depart if a new deal isn't secured.