Unlocked 2 rivens (primary+secondary) with same challenge in one mission: one unlocked, the other disappeared?

[Yggdrazzil]

They both were 'collect 8 syndicate medallions'. I completed the mission, got to the "riven unlock" screen where it showed only one riven (afuris) was unlocked.

I checked both weapons in the arsenal, neither of them have a riven installed anymore.

I checked my rivens in the general mod menu, sorted by 'recent' and only the afuris one is there. The one that unlocked.

Where did my other riven go? 0.o

[CaptainPancakes]

I thought the same thing was happening, but I was equipping the same riven on two different weapons. I took kitgun and zaw rivens with the same "kill 4 sentients" challenge on a mission just now and both unlocked. 

Edited January 6 by CaptainPancakes