Repro steps:
1. Using the Terrain Toolbox (Terrain Utilities), export two distinct (make sure they look identifiably different) heightmaps from 2 different terrains, one to "raw" and one to "png"
2. Go to the "Create New Terrain" tab, make sure "raw" is unchecked and choose your PNG heightmap as a texture
3. Check "use raw file" and choose the file you exported previously
4. Create your terrain

Expected result: the created terrain uses the raw file

Actual result: the created terrain always uses the assigned Texture2D when Raw file mode is unchecked, unless that is set to "none"