There is a zombie apocalypse in the normal day of the perfect passive timeline. The Sanses are so confused about what the zombies are and what to do so that's why they all cling to their Charas not Frisks as the Charas know more about the zombie apocalypse than Frisks who always been with the toriels. The Sanses also have codependent on their Charas too so it's not a surprise if they choose Charas or priorities Charas over Frisks as it's in their codes.