Wild guess, but maybe they are monthly?
it took 6 hours of NA Maintenance this week to make this chance? That's a lot of work for this just update!
Seems that some rares are even daliy, but not 100% sure.
Something to do with the bug that also reset weekly Delves quest soon after completion or next day for some? š¤
This must have been a bug that only affected some people because I've killed and gained the rep from every one of those rares except for the bird at least twice and most many times before this week.