I have this same issue. The initial Shrieker in Act 1 that I encountered was visible and clickable, but at some point during Act 1, they all became invisible. Judging by reddit/steam/here there are a bunch of people affected by this.
I'm kind of scared to click through into act II now, because if what people are saying is true, once you render the Shriekers once, they'll vanish on any subsequent save game loads and be unrecoverable.
