Never heard of that bug before. With the SMW bosses, you can usually fix graphical glitches by changing it from "Normal Speed" to "fast speed" for a second, then switching it back. In v0.5 Beta and older, this option is "Wait Vblank" and "No Vblank"
EDIT: you can also try the "fix graphics" button. In v0.5 Beta and older, this is in the GFX Config. In v0.6 Alpha, it's in Advanced - Graphics Config or something similar to that (don't have the menus right in front of me so I don't know exactly what they're called).