PlgBlt() function is showing ghosted image of bitmap mask..?

Hi all,

Using the PlgBlt() function to draw rotated bitmap of a sprite to the screen.

https://docs.microsoft.com/en-us/win...-wingdi-plgblt

I'm specifically using the paramater [in] hbmMask to provide a transparency mask for the bitmap.

The problem is... when I rotate the image 180 degrees while the program is running, the transparency mask suddenly becomes visible (looks like an opaque, white image of the sprite), rotated into its starting position (of 0 degrees).

When I rotate out of 181-179 degrees, the bitmap mask disappears again and the sprite renders correctly, transparency and all.

Any ideas what's going on?

EDIT: This is the exact same problem this guy is having in this thread from 1995(!).

http://www.verycomputer.com/418_bbc47825d091af69_1.htm

Surely someone has determined the cause of this error since then..?