I'm hoping someone with some advice replies soon because I was just wondering the same thing. Did a baby hack job on my hair in December to cut out some color. Majority of it is cool but some areas are noticeably shorter smh
The only way it will "catch up" is if you trim the longer hair consistently over some time.
Chyme(Hair Crush) shaved the side of her hair and has been growing it out for a year+ now. It's long and has grown out a lot, but it's not anywhere close to the length of the majority of her hair.