Scars in One Piece have special meanings. It is a reminder of the conflict of the character.
Luffy’s iconic scar in his face is a reminder of Shanks, his attempt to prove his worth and eventually his promise to Shanks.
Zoro’s scar across his chest is his reminder of duel with Mihawk and the promise of an eventual clash.
Shanks scar was given by Blackbeard and this leads to the theory that they are eventually gonna face off.
The scars that has resolved its conflict disappear or the characters don’t have a scar at all. Take Luffy’s fight with Katakuri. He was missing a chunk of his flesh but it didn’t leave a scar. It was a lot more brutal than him stabbing himself but there is no hint of injury. It is because there is no more conflict to be had with Katakuri anymore.
There are many instances in the series where there is no visible scar even though there has been a violent exchange and this is my reason as to why. I recently read Rurouni Kenshin and I think there is a similar idea in the series. Oda worked as an assistant in that series so I am inclined to believe Oda was inspired by the series to do so.
*Theory by Boundman101