The Texture Compression labels are misleading, they should follow a certain pattern, maybe showing a Tooltip.
“Greyscale” for example is uncompressed, but there is no indication that it is. Instead you should use “Alpha” for compressed greyscale textures, although your maps aren’t used as “Alpha” maps.
The Default Texture Compression setting says “DXT1/BC1 DXT5/BC3”. However, the compression of the Alpha channel of these 4 is completely different (DXT1/BC1 with 1 bit Alpha and DXT5 with interpolated values and BC3 with more bits than 1). What decides how the Alpha Channel is compressed?