All drop rates, dragons from the Soulforge, mythic troops in chests, hoard mimics in explore, all data is stored on our servers and not individual player accounts.
If we were to experience any issues with the drop chance for any of these, it would affect all players, as all data is being pulled from the same place. So, no players would be seeing these troops be rewarded.
There are already changes being planned for Underspire to improve it based on player feedback and one of them is to address how difficult it is to earn these troops because their chance to be rewarded from defeating a Treasure Guardian is so low.
But in the meantime, it is not a bug.