Revealing mistake: When Maleficent freezes the people in the church, the actors can't help moving.
Continuity mistake: When Maleficent places the glasses on Beast's face, their position keeps changing all the time.
Continuity mistake: When the movie begins, Mal's hair swaps between being dark and light purple.
Audio problem: After Ben eats the brownie Mal gives him in the carriage, you hear Ben say Mal's name but his mouth never moves.
Answer: The first Descendants is unclear with this information. SPOILER WARNING. In the beginning of the song "Queen of Mean" from Descendants 3 we see pictures on Audrey's mantle with her and Ben as little kids. So we can assume that they have always had some sort of close relationship, but it still unclear as to when it was official.