Anakin in fact should be expressive, but also have a tendency to repress his emotions in distress, forming a cold persona. This doesn’t mean he can never fly off the handle either. He goes cold because he cares too much.
As I’ve noted, this is shown in the OT. Vader regresses into his cold persona immediately after he admits something vulnerable or Luke says something that gets to him. In Anakin’s death scene, now having abandoned the dark side, we see he is very caring and sentimental.
In the Prequel Trilogy, Anakin was in his early 20s, while in the Original Trilogy he was in his early 40s. It is natural that his behavior and mannerisms would have changed over time. Twenty years separate the events of these two trilogies, and it is uncommon for individuals to retain the same attitudes and ways of behaving in their 20s as in their 40s. This is especially true in Anakin’s case, given that he fell to the Dark Side and became a Sith Lord. It is entirely realistic for a person to evolve over such a span of time. Thus, Anakin’s transformation reflects both the natural passage of years and the profound personal experiences he underwent in his journey from Jedi Knight to Darth Vader. Furthermore, I do not remember any particular instance where Vader represses his emotions in front of Luke. Could you provide some examples?