That’s odd, since it works fine for me with 4.6.1… Hell, I’ve been using this method since 4.1 without any issues.
Are you importing the RAWs as layer maps when building the landscape, or are you trying to import them as image maps? I import the RAWs WITH the Landscape at generation time. I haven’t heard of PNGs working with this method, so I can’t attest to it’s usability. Using PNGs for materials, yes, but this process is for getting the layer weights imported into the landscape itself, rather than as a material.
Here is a visual representation of my process:
