It doesn’t occur in 4.26.
The problem is with UE5.
The strange thing is, that the seams have nothing to do with the skin textures or normal map, but with the character (seams are at all uv map edges in lit mode; in unlit mode it’s ok) itself, when skincache is turned on (which is absolutely necessary).
No recalculating of normals, no higher precision uv map neither changing the “(bFullPrecisionUV)” code has helped so far.