This armour being different from Mando season 2 repainted version; different paint, scratches and damage, different undersuit etc
So somewhere between taking the throne at the end of Mando season 2 and the start of BoBF, Boba repaints his armour, mounts it on a new undersuit, then gets into some kind of battle/adventure that results in damage and chips to the paint on his armour, including that "cut" scratch across the right side of his helmet.
I'd rather watch that sequence/explanation than the BoBF we got.