Within the last 8 hours of gameplay (after checking all of the time stamps on my available saves), Eder just vanished from my party.
I initially swapped him out to play with another companion after completing his companion quest. After many hours of gameplay, I progressed Maia's quest, in which she leaves the party temporarily and becomes 'busy'.
I then noticed that I could no longer recruit Eder. He was also no longer below deck near Aloth, and any reference to him vanished from my Reputation page.
Unless this is a bug, the only possibilities that I can think of is that he possibly died during a Ship Boarding combat sequence (is this even possible?), or that he got annoyed because I killed whales during sea encounters (he is an animal lover). I never got any notice that he ever left, or even died (At least I think not)
Can anyone shed any light on this? I really don't wish to go back to a save file from 8 hours of playtime ago and repeated everything.