Info derived from the LV Champions forum from users thoric and intaris

The transparency of icons is determined from the four corners only. So, if the four corner pixels are non-white, it doesn't matter what else is white, it won't be transparent.

If there is any “non-white” border which fully encloses an area of the icon, that area will NOT be transparent, irrespective of the colors within.

Poke a hole in that wall with a single “white” pixel and suddenly the adjacent white becomes transparent also.