Some hatchlings come out this way, but it usually fills in over time. I don't know this species well, but what has the diet, calcium and UV routine been? Does the carapace feel soft or squishy? The plastron?
He does not feel soft at all nice and hard, UV bulb is on all day and I use a Che that stays on all the time. Diet is great he eats everything I give him now all greens, different fruits sprinkled with calcium and also protein once a week. he is eating drinking peeing and pooieng well !! And has a soak everyday.
That really is interesting, because the exact indented line is where the spine is fused to the inside of the carapace. If anything, you would normally expect to see a slight raised line on that strip.