Same here. I have all twelve hunter masks but only six ivory keys. I think completing the hunter encounters in other players sessions causes a malfunction. The session owner gets the ivory key with you only acquiring the mask. Or it’s that the ivory key just fails to drop regardless of the session you are in ( your own or another players). Hopefully this gets fixed...
https://imgur.com/gallery/kgO2nlY
I was told via ubi support that the ivory keys are not a guaranteed drop, see the link above to see my conversations with ubi support
https://imgur.com/gallery/XvMAqtc
More run around from support
@ SctGabrielDK
i gave it up to unlock an 8th key by Massive.
Since the keys are not savegame but character-bound, I created another character and had all 12 real hunters killed by a friend. So I finally got my 8 keys now.
I find it really weak of Massive that they simply can not manage to connect a "trigger event" with the ingame statistic.