This "issue" has been discussed many times in the forum. This behaviour is intentional, it is done so to prevent displaced mesh tearing at the sharp corners. To make this displacement skewing less apparent, you need to add more subdivisions to the source geometry, especially near the sharp corners. It's a similar concept to mesh smoothing - if you'll add turbosmooth modifier to a cube with no additional segments, then it will round that cube to almost a sphere, but if you'll add few additional segments, the cube more or less will retain its shape and the closer lines will be to the cube's edges, the more sharp it will be after turbosmooth. The same is true with Corona displacement, except it's not about smoothing, but about skewing. Hopefully you get what i wanted to say. In conclusion - this is not a bug, but a feature.
P.S. one of many past discussions about this on the forum:
https://forum.corona-renderer.com/index.php?topic=38964.0
