I think because the orb that Hyo-un fed him wasn't complete yet so as a result he reincarnated as his mate but with a memory loss
Yeaahh I think so too
from what I understand the reaper said that Taehyuk is destined to die at the age of 33 (on all his lifetime ig) Tae's soul is about to crossover but bec Hyo-un gave the orb it prevented him from crossing over. The reaper there warned him that there are repercussions of his actions and maybe that's the why he lost his memories.
I think the logic why he lost his memories is bec whenever Tae dies in that lifetime and be reborn again of course his past memories are no longer intact. So since Tae techically died (is about to die) he is already like an empty shell (probs being ready to be reborn again) thus the memories where already wiped out when his soul was forced to go back to his body
I'm happy that we'll be getting the present time again but sadly Taehyuk has some kind of memory loss :(