People often complain when a character who had gone through an arc seems to have lost all development, and I get it, but we also need more non-linear character development. Not everyone only continually gets better. Most people will slide back or relapse. Let characters struggle to improve.
It’s more realistic and lets people see how difficult changing is, whether it’s changing from a bad person into a good person or someone healing from trauma/depression.
Also, sometimes things happen that cause a relapse no matter how well someone was doing.
It’s complicated.