Blizzard
We've addressed an issue that was preventing some loot from being tradeable. If you've been in an instance since before the fix (at about 18:40 PST), you may need to leave and reset the instance manually.
In the case of Mythic Raids, wait 30 minutes outside of the instance while making sure nobody remains inside or goes in early, before going back in. This will not make loot that dropped before the fix tradeable, but all further drops should behave as expected.