Blizzard has confirmed that players not receiving a Resplendent Spark after defeating Uber Lilith is a bug and will be fixed with the next client patch! With Vessel of Hatred revamping Mythic Unique crafting and decreasing Mythic Unique drop chances, Resplendent Sparks are even more scarce.
Hopefully, players who defeated Uber Lilith this season will be rewarded with their Resplendent Spark with Patch 2.0.3!
Uber Lilith Boss Guide