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.
The addon Silver Dragon checks if rares will drop rep when killed. Using the addon means you will always know when these flags get reset, or if a rare that was originally set to be a one time rep rare turn into a weekly rep rare. Super useful!