I just had a crazy idea... what if the original Bungo was cryogenically frozen? It would explain why Wakki kept aging and yet they were hito-narby'd together. Assuming the hito-narby-have-implanted-souls theory is correct it could be that they extracted it from the comatose frozen Bungo and that's why she has memory issues. Her hito-narby body can't support her under these conditions.
I just imagine Wakki finding her cryopod in the future and getting absolutely mindbroken.