Players on Reddit had quite a good thread about this:
Reddit Thread on Bugged World Boss Completion Diablo 4 World Boss Overview
Make sure you are staying until the quest tooltip updates as you can see in the video below.
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Season Journey World Boss and Tree of Whispers Objectives Bugged – Completion Not Triggering
Have you noticed that no matter how many world bosses you fight, your battle pass just won't update? Players have found that you must wait until the world boss achievement completes to get credit on both the battle pass and the world tree whisper progression.
