Pretty sure that's why
It seems like Amon made a deal with the priest to erase na-in's memory through a ritual (the poison room) everytime he gets suicidal from the killing of humans. Amon gets a "happy" na-in and the priests get a "god" whose power they can subjugate and use to gain power and control over the people.
apparently (massive spoiler)
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amon’s a dragon & na-in is his mate but he lowkey can’t handle immortality so amon erases his memory regularly. that’s why amon says “the first time i saw you you had a feather in your hair” and na-in does not remember it…cuz it happened A LONG time ago
Oh damn thnx .. I was getting frustrated with long ass waiting game
Soooo we aren't getting no fucken answes yet as too why he is forgetfull?? Someone spoil it for me aside from him going to those chambers with the smelll of poison