Landscape painting makes squares

Hello everyone !
I was testing some landscape painting to see if the textures would fit nicely for the world i’m imagining, but i have encountered an issue.
When i paint on my landscape (using a layer blend) it appears i have “squares” and it’s not smooth at all. I have tried only using 3Vector for simple color and i have the same result.
The fall off brush doesnt change anything and i have my landscape resolution correct according to UE doc.
But if i use one of the bridge texture for a base color the problem is gone !?
So i would like to know if you know what i’m doing wrong or what could help me fixing this.
Have a good day !
And some pictures here to explain my problem better :

(And i use substance painter to make my own textures)
And i checked all the other topics but none solved my issue :\