I am sure I'm not the only person who was clearly reading this book waaaay too late into the night to notice one of the big reveals mentioned
Yeah, I really have to blame the late night brain fog because anything else would be inexcusable.
Crazy formless theory:
We know there’s unmade influence in Shallan’s family (or, at least, it’s implied). What if Shallan is an unmade and formless is the unmade personality that she’s suppressed.