I found a sneakier bug on day 12. Even though I got every mix right, my score at the end of the day listed 8 potions given, 5 correct, and 0 incorrect, leaving the tally on the pause menu at 5/8. It almost certainly has to do with the Bimbonomicon sequence overriding the potion effects, but it doesn't seem to still check for the correct formula like it does on day 19. This sticks out a bit more since your score on the previous days only goes up to 6/6 or 5/5 to keep the vampire happy.