I just reached T20 Coven Favour by grinding the same 4/5 activities for what seemed like a millennia.
The T20 reward states “three guaranteed mythic items and a resplendent spark”
I got the spark, along with one Mythic Kursat Tribute, but no actual Mythic items.
I used the Tribute in the Under City, achieved the rank 4 rewards and also got zero mythic drops.
Really feels grim to finally finish the grind and get nothing. A single spark on its own isn’t even enough to craft a mythic.