Painting a landscape changes material of entire section


I have a problem visualized on the picture. When I start to paint a landscape (from landscape blend materials) it chnages materials of the entire section on which I paint (Getting bit brighter, difference is small but visible upon picture enlargement). Plus every time I paint something on my landscape the “Apply” button in the landscape material blend gets unchecked and green as if I introduced some changes to the materials.
This brighter section is also visible in the game.

Is there a solution?

[EDIT] I might add that I see this effect only with certain painting materials, not all.