Blizzard has stated that players who defeated Uber Lilith but didn't receive their expected Resplendent Spark will have that granted to them in Diablo 4 Patch 2.0.3!
Patch 2.0.3 Notes
Similar to an issue in Season 4, players began reporting that slaying Uber Lilith wasn't dropping a Resplendent Spark as the fight should. Thankfully, Blizzard are addressing this issue in the next Patch 2.0.3, releasing on October 17:
Keep in mind that players who experienced this bug in prior seasons had to be wary of which character they logged into immediately after the bug was fixed, as the Spark would be awarded regardless of the Realm mode they logged into, sometimes locking the Spark on lesser played characters (such as in Hardmode). To be safe, make sure you log in on the character you want to use the Spark on once this fix goes live!