Hard to say with certainty.
Top issue givers: Weight Paint, Bad UV map, bad material usage, no smoothing.
Also, the way things render within blender or any other 3d program is not an accurate method of making any kind of determination when it comes to the engine - basically, it doesn’t matter what looks fine in a 3d program that has no sorts of rendering constraints, and comparing one to the other is a bit worse then apples and oranges.
Make sure you FBX export is correct and that it doesn’t mistakenly contain the high poly. I’ve personally done that before.
Make sure your file has SMOOTH shading - enter edit mode, select all, drop down the Faces panel and select smooth - make sure you export correctly selecting “smooth” faces.
I must note that it looks like the normal map is not a complete mapping in this picture. it seems like High detail in the middle and lower detail below/above the seams. basically a body should have no seams (the unwrap is not too complicated. mark a seam under the foot/palms, mark a seams on the inside of the legs all the way up. ride the seam mid-body all the way up to the cranium. then for the arms you split with another seam riding up to the armpit, at which point it half-circles to connect to the middle, in somewhat of a T shape near the scapula.)
I’m currently having issues with the Mips causing visible seams. and this is happening despite the diffuse texture being specifically created with the background color extended well beyond the area of the seams.