devtrackers.gg
This isn't another "I got a Very Rare I already had" post, and this has nothing to do with the Nontradable item bug.
This video literally shows an Uncommon Drop giving a Common Antenna, something that shouldn't even be possible.
The most likely cause of this problem is because Rainbow Flag was previously an Uncommon item, but was changed to Common and given to everyone.
I'd wager that they simply didn't remove Rainbow Flag from the Drop table when they changed the Quality.