This isn't a bug, people have been using Three of Coins as an exploit to get exotics fast, In the latest patch Bungie, have fixed this:
Three of Coins now provides a smaller increase to exotic engram drop chance when used in rapid succession.
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That was like 3 months ago, I know a out the exploit, I've used 57 one at a time and gotten one exotic when just last week's I would get an exotic like every 5 or 6