A 27-year-old British soldier returns from a 6-month deployment in the Central African Republic. He was taking mefloquine prophylaxis but stopped 1 week after leaving the endemic area instead of continuing for the recommended duration. Three weeks later, he develops fever, rigors, and headache. Thick film shows P. falciparum rings at 0.5% parasitaemia. He is haemodynamically stable with no features of severe disease. What is the most likely reason for prophylaxis failure?