Apple wrote:
The angel hidden tier 3 is not working, wrath and human hidden ones are just working fine like the guide
According to the guide on the Masin board, the "Trial of Decay" which is required to get Angel Hidden Character is described as follows: "The way to death, is by none other than those that harbor itself..." without any specific location or other information.
I haven't gotten Sitael personally on Afterbirth, though since the others tier up in the same spot as they used to I assumed it to be the same with her. Maybe you still need to bring an Azrael Spirit with you? I'd heard you didn't need to anymore, but it may still need it. Hopefully you can get her now, or already have!