If you upload that terrain, I can check what it wrong.

Here's the remaining info - I'll also add this to the documentation:

HMP5_HEADER::flags determines whether the terrain is a floor or a cave ceiling, i.e. if the normals point upwards or downwards.

And the M7SKINTYP_RGBFLAG differentiates between RGB and BGR bit order on 24 and 32 bit skin types.