Also (felt worth making a separate post)
I was fighting the bunny-guy boss, and used the syringe to heal right as my toxin meter filled out, causing the screen to go black but not causing a game over. I could still hear game audio so I just mashed attack buttons and eventually it skipped to the final victorious cutscene. (I'm not sure if I was simply invincible during this bugged black-screen state or if I just got really lucky)