Rare Crests only have a 5% chance of generating a legendary gem drop (and it’s always a 1-star if it happens) which is why the event information and the patch notes all tell you that you should use legendary or eternal legendary crests as these guarantee a legendary gem drop per crest.
The event not working with rare crests is not a bug. It’s by design. It’s why Blizzard gave everyone plenty of notice about using legendary / eternal legendary crests for the event, to maximise the amount of bonus gems you’d get.
From the patch notes
