Question "Cubic" effect when blending two terrain textures

Hello!
On Unity 2021.3, I recently changed the a Terrain Layer already painted on my Terrain Object (from grass to snow). To do so, I directly changed the Material and Normal map to new ones, without repainting everything.
However, it seems like it doesn't fit well with the rest of the applied textures. See the difference between a texture blending rock-snow and snow-soil :


As you can see, the first one is "smooth" (I don't know how it is supposed to be called) (which is normal) while the second one is "blocky" (which isn't normal).
The mistake seems to affect every new added textures as well. So if I add new rock texture, it will have this "blocky" effect as well.
This effect only applies for textures added on the "snow", so the modified texture.
Does anyone have a solution to this ?

Thank you!

(edit: I updated the post with newer information)