Just read this one recently!
https://www.mangago.me/read-manga/torezoa_boku_no_takaramono/
Here you go! White haired elf wipes memory of him sleeping with the knight but Knight remembers eventually because he instinctually remembered the feel of the elf 's body and knee it couldn't have been the first time. I remember reading it and going this is a bit more angsty than I was expecting!
THIS IS THE ONE!! I’m actually such a fiend for memory-loss angst so thanks for finding it for me
No problem, happy to help!
I read this fantasy (6ch~) black and white bl manga a few months ago where the hero and the elf set off to find more elves after defeating the demon king. The hero was in love with the princess (who got married) and the elf is in love with the hero (erases his memory of them at it)