raster edge normalization

[dorinoltean]

I have a raster that covers a section of the map. The raster is does not cover a full tile [x,y,z] so the libray fills the remaining space to cover an entire tile. (sky blue on top is what rio-rgbify adds)

The transition from my raster, to the fill added by rio creates an unnatural edge to it which in a hillshade on the map looks like this:

The original raster does not have that intense margin.

Is this the expected behavior ? Do you have any suggestion how to avoid the prominent edge ?