Book!Alec only knew of Magnus’ past relations with Camille from word of mouth yet took this as enough evidence to slander Magnus’ character and become completely distrusting of him for two books (give or take).
Show!Alec saw Camille actually kiss Magnus when they were just getting started with their relationship but had the common sense to recognise that it was against Magnus’ will and gave him an opportunity near the end of the episode to explain himself before jumping to conclusions. Show!Alec also didn’t feel threatened or uncomfortable when Magnus relayed to him how he had struggled with turning Camille in to the Clave because of their past and encouraged Magnus to be open and honest with him about everything, including his past relationships with other people.
Jesus, take the wheel! This show is just too good. What did we do to deserve this masterpiece? :’)