I finally got what "erased"/killed Tsukasa (spook 131 spoilers)
Amane and Tsukasa were transported to adult Amane who became possessed in order to convince Tsukasa to go into the pit and ending up killing Tsukasa. Then, Tsukasa wasn't properly killed because he didn't consent so Amane's health deteriorated and he died (which happened when he was a teacher). This is because the failure to get consent for the wish led to a failed sacrifice and the wish being undone. The medicine in chapter 119 supports the theory that Amane's health deteriorated at some point.
Please tell me if there are any holes in my theory or if you have anything to add.