But Fred, making his 100th Premier League appearance, was in the right place at the right time just three minutes later as he pounced on a rebound from opposing goalkeeper Kasper Schmeichel and levelled the scores.
Leicester looked to have scored a winner on 79 minutes when Maddison fired into the bottom corner, but, following a VAR check, referee Andre Marriner judged Iheanacho to have committed a foul in the build-up.
"It's one of them: you've seen them given. I'm obviously going to be biased, I don't think it's a foul," Maddison said.
"It was a great feeling for about three minutes! But as soon as you see them going over the monitor... there's no point them even going over any more. Even after that, we had chances on the counter, but we'll take a point."
United had only one other real chance when Fernandes, running through on goal, was denied by Schmeichel.
United, who were without forward Cristiano Ronaldo due to what the club said were "flu-like symptoms," remain in sixth in the Premier League, with fourth and fifth-placed Arsenal and Tottenham Hotspur both with at least one game in hand.
"Everyone gave everything," Fernandes told Sky Sports. "I don't believe there's anyone who didn't give everything in the game. Nobody lacked intensity. We have to keep fighting. As I said before, we have to keep looking at our own games."
Information from Reuters contributed to this report.