I don't understand anything about this issue, but something is very wrong. Attaching example. In it you'll find the following:
1. An image drawn with Screen.DrawTexture. That one will be visible right away.
2. Type give testweapon to get a test weapon using the same graphic. Note how it renders "broken". Try gl_mask_sprite_threshold 0 to see how it changes.
3. Type summon test to see the same image applied on an actor. Note how it renders differently.

My assumption is that gl_mask_sprite_threshold should not be applied to PSprites using this renderstyle at all.
Edit: removed information about image being "subtractive," that was my mistake when I applied black fillcolor, and streamlined the text.