They are very rare. I have only gotten one so far and have done each day Hard and Expert (some days hard some days expert depending on how hard as all I have are R4 4*s).
Ive gotten 3 so far. 1 a day for the past 3 days. I’ve been going with the 3rd level difficulty (4* shards difficulty). Gotten a few good pulls as well. Just grabbed another 4* Hulk today from it to max out his sig ability.
Comments
How does this qualify as a bug?
Not really. It is a random drop. I have yet to get one, but have seen lots of people get exactly 1 so far. They are not meant to be common.
EDIT
https://forums.playcontestofchampions.com/en/discussion/comment/297894#Comment_297894
Miike even chimed in about it being a rare drop.