Or the person was not taking PrEP as prescribed, either missing doses or taking it inconsistently. Once they're infected, if they keep taking just Truvada, then that strain will begin developing resistance, which they can pass on, potentially to others taking PrEP correctly. There have been I believe 6 cases of people being infected with resistant strains while in PrEP.

That's why it's important for people taking PrEP to be screened regularly, and put on a proper cocktail if they test positive.