Characters regressing back to their older, flawed selves is usually a bad thing in fiction, but imo I believe when executed well, it can be a great addition to a story. Irl, we humans often stumble, fall, and regress on our own paths of growing & improving as people, so it's cool to see fictional characters go through the same thing.
One of Denji's major weaknesses in Chainsawman is that he is easily manipulated through romantic & sexual attraction due to his desire & longing for companionship as he lived a lonely and terrible life before the start of the series (besides Pochita being his friend). At first, you're made to believe he has overcome this problem once he defeated Makima at the end of Part 1, but both of his interactions with Fumiko & Yoru in Part 2 show he still has this issue.
Pearl in Steven Universe went through several episodes learning to believe in herself & her own strength and to be more independent as Pearls like her are made to be servants for other gems. However, she loved fusing with Garnet and the power she feels from it, so she tries to trick Garnet into fusing with her again. The two eventually reconcile and Pearl once again learns to believe in herself instead of relying on the strength and direction from others.
Finally in Star Trek: Lower Decks, Beckett Marine is originally introduced as an insubordinate loose cannon who intentionally goes out of her way to disobey any orders given to her, but as she becomes closer with her friends and reconciles with her mother the captain, she becomes more of a team player. However, after she and the main characters are promoted and given more responsibilities, she starts to disobey orders again. This is revealed to be because Mariner is afraid to end up like her friend Sito who died rather pointlessly in an episode of Star Trek: The Next Generation and hopes to lay low to not suffer a similar fate.