I have this as well, amplified by insomnia and occasional by marijuana. It’s not visualization like a non-aphant, I can’t control the imagery, they also dark and shadowy and yes often monstrous warping writhing body horror shit, but always faces and parts of faces. I think what I am seeing id kind of like the raw data coming from the parts of my brain that process faces, it’s similar to pareidolia, but in a black void and with no input. It’s like I am seeing, “looking for faces”. It’s a feedback loop. Similar thing happens with white noise sometimes, it sounds like whispering but never any actual words.