kinda spoiler
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Because Eyra went to Labyrinth! I forgot whether its already explained or not in manhwa, the Labyrinth has defense mechanism including hiding aura? Smell?? Kind of that. Thus when 6 y.o Eyra went to Labyrinth through special portal, his aura also hidden by the mechanism so Yanu only could detect his last trace on Solath. This why Yanu insisted before Eyra couldn't be his mate because Yanu should detects his aura on South (Labyrinth) but turns out there special mechanisms :3
I read later in the novel later... yup. -_-' if not the funeral, he would have been hidden forever.
Yanu didn't realized his mate because he only awakened as a reincarnated when he was six?