Really weird. Smear under 0.5 strength doing nothing and over is only at one level. Like its only on or off. And colors seem not too varied. I would almost guess that its displaying only 8 bit colors only or something.
It's definitely only displaying 8 bit colors by default. I discovered last week that we should be enabling the 32 bit float operation when creating textures in 2.8, especially for use with Eevee.
As for the smear brush, strengths lower that 0.5 are still effecting the texture for me.
Perhaps the color values are so similar in your texture that there's little perceptible change?
HI Kent, thanks for your tipp, because I had problems in painting. After creating an new image with 32 bit float I can paint perfectly. Thanks a lot.