I didn't make it!
There are brushes throughout my map that have a side completely invisible, even though it's the exact same as the other faces around it in Radient. Is this because I'm using 1.5, and how may I fix this problem?
I think I saw a post similar to this a while ago, try copying the brush, deleting it and pasting a new one there and see how if that works, sometimes brushes mess up.. in my experience any way.
Making it detail A brush cleanup
And if those fail, rebuild it.