[not a bug]Visual: BFG shots transparent when invulnerable

[TheDarkArchon]

When you are invulnerable, some areas of the BFG are translucent.

[Graf Zahl]

That's because these areas are white in the original graphic and hence black when inverted. But when using additive translucency (as here) anything plus black looks like the black is translucent.

You will see that ZDoom's software renderer has the same problem.

This is not a bug, it's a limitation of the way translucency works.