We can be almost certain it's a scar because of the way Nami asks where Sasha got it (assuming the scar because of the images and the way Nami worries it is a sensitive topic) and when Ayua touches the scar and says "I know you don't" hinting she knows how Sasha got the scar.
He had better ones in the old Wiab, when Willi was in hospital after being beaten up, and when Katie got scratched by the raccoon.
But maybe they were too generic for what he had in mind for Sasha's story, he customised it early, and now it's kinda like a birthmark even if it's a scar - would we think of her as the same character with a different scar?
Nika's fight scar looks odd, but more believable, and was developed at a similar time, so I'm just going to assume Ocean wanted it to look a certain way. In some angles, it looks like it's meant to have stitches, suggesting it is recent (which might be why she has Inessa as a bodyguard):
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