terrainData.heightmapTexture float value range

Correction: Not a bug. It's a feature :

"The heightmap implementation itself is signed but is treated as unsigned when rendering so we only have half the precision available to use for height values. That's why all of our Terrain painting shaders clamp the returned value between 0f and .5f so that we don't end up writing signed values into the heightmap. If you were to put in values greater than .5, you'll see the Terrain surface "wrap" to negative height values. I can't say why this was done but it probably has stayed this way because it would take a lot of code changes to make either of them signed or unsigned to match.

The values are normalized so that we can get the most precision we can out of the .5f for a given Terrain's max height. 0 being a world height offset of 0 and .5f being terrain.terrainData.size.y (the max height)"

SOURCE