Catbox.moe blocked by xfinity advanced security.

Why can comcast tell what site I want to go to when using Mulvad? Catbox.moe shows up as blocked in the xfinity app everytime I try to connect via my wifi connection with Mulvad on. It works fine when I use mobile data instead, so theres no other factors blocking it. I thought by using Mulvad the isp wouldnt know what sites im going to?