Thats just how the landscape works.
You have 1 vertex evry unreal unit (which is the same as a meter).
The resolution you have to change would be tjat of the landscape.
Common ways to lessen the effect are to filter the edge by some other texture.
Its also why your height blend looked slightly better.
Add some other texture to break up the linearity of the paint, use bigger paint brushes that cover areas larger than 1 meter.
Etc.