Update: and the hotfix has arrived!
Blizzard is currently testing a fix for a Tree of Whisper rewards bug that affected players last night.
Global Community Lead Adam Fletcher today announced on X that QA is testing out a fix for a bug that caused players to be unable to obtain Tree of Whisper rewards. The bug specifically affects players who played between 12 AM – 3 AM PDT and progressed through a Tree of Whisper turn-in.