The problem is that the seam is not in the middle of a a normal map piece in the texture. my seams are all in the borders !
And i use symmetry !

In fact i found some to lower the visiblity of the seam :
I painted on the 3D object in it's normal map channel where the seam to rasie it a little then used some smooth on the normal i painted !

It is less visible , but to be perfect perhaps i should paint
on the normal map directly ?
The problem seems to be the normal map height going down near the seam !

Strange, but now it's lot better, so i'll keep normal map that brign some good volume information also