Mythic Packs can contain Rare Wildcards in the Mythic Slot
Mythic Packs can contain Rare Wildcards instead of a Mythic Rare card (this should at the very least be a Mythic Wildcard)
My collection has Mythics from that set I don't yet own, so it's not an issue in regards to that, it could be some cards are missing from the pool and that's the issue? Happened twice, the first time was closer to the release of Mythic Packs and assumed it was a bug then as well, but if it's still not fixed this is a major issue.
If it helps Draconic Destiny, Platoon Dispenser, and In the Trenches where the Mythics from Brothers War it should have selected for - if for whatever reason those cards aren't checked when trying to generate a Mythic from the pack that could explain this unintended behavior.
I'm assuming it's unintended behavior because the alternative is disgusting misleading and malicious behavior.