I think this could even make sense inverse. Like luffy growing back his teeth.
We now know Luffys DF is the Nika DF, which is the "most ridicilious power" and "could do anything the user could think of". Which pretty much explains Luffys Rubber Style fight and why it sometimes even didnt fit the parameters of rubber. Because it was the Rubber style how Luffy has imagined it. Luffy thought (!) and believed thats how rubber works, thats why his fruit worked like that.
Exactly the same as Luffy thought Milk will heal his tooth and Meat will give him energy and strength straight up back. Because he truly believes in these "facts", they become real through Luffys DF.
And i think the same is happening with luffys bigger wounds. Luffy thinks they just heal away and they do. He just believe it like that. The difference with the Scar of Akainu is, the scar is also emotional (Aces Death), so it wont heal as long as it will hurt. And this scar will hurt him forever.