Limit Adult Websites - major problem

I had selected Limit Adult Websites for my son’s iPad. We recently discovered he had been accessing large amounts of adult websites and I couldn’t believe they all got past that filter. I went back into my family controls and Limit Adult Websites was still selected, but every single adult website he had accessed was in the allowed Websites list. It’s as though they automatically appeared there when he accessed them. This is a major fault. How did this happen? And no, he didn’t add them using my phone.