shell question

Stephen, I’ve been a member of the forum for awhile but haven’t posted in ages. I have a 3 year old diamondback with florescent, UVB/UVA lighting, good filtration and a varied diet of fish, pellets, cuttlebone, and crickets. I inspect his shell and skin regularly and I’ve noted in the past that in the creases of his shell between the scutes sometimes there is a thin line of lighter colored shell in very small sections that appear a little softer than other sections of shell. I figured that it was where the new shell was growing from and the vet never thought anything of it either, and in time it was the same color and hardness of the other areas. Recently though I noticed a thin line on his underside (belly for lack of the right term) right in the midline crease that appears a little lighter and has a little give when I push on it. It is a linear area less than an inch in length and very thin, corresponding to the line that runs from his head to tail underneath. Does it sound like just new shell forming or am I completely off base??? His lighting is changed every 6months to keep the full spectrum of wavelengths and he’s been eating cuttlebone regularly so I can’t imagine its due to poor diet/calcium. Also it is only in the thin crease and doesn’t appear to be shell rot. Any comments would be greatly appreciated… Thanks in advance!!! Jason