Image to Material + Preserve Seams makes Errors in the Highmap

Hello, while trying to make tiling textures for a videogame, I've encountered this issue:

Image to material leaves serious gradients on not only the four sides of an image, but also in the middle.

Obviously, I've activated the Preserve Tiling Button. While it is not technically a seam, the heightmap looks bad when applied on an object, because of the deep crease created by the software.

Interestingly, this only happened in the deep/dark areas. The raised areas are basically perfect.

Different settings in small/med/large details did not fix this.

Logically, the central gray zone should have no large gradients. We can even see 45 degree pale lines, as if this was some kind of bake on a sphere.

There seems to be no options to control the Preserve Tiling. So I don't have other levers to change the behavior.

1. v 3.41

2. PC, Windows 10

3. Image To material with a texture with dark zones, activating the Preserve Tiling

4. The flat areas should be flat in the heightmap, instead of having strong gradients near the edges of the image.

Am I just silly and forgot some option, or is preserve tiling not fully fonctional?

Thank you