Hey,
I assume you are using Blueprints.
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Make OnChange dispatcher, function or event.
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Call it when your healthbar changes.
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Make disappear animation (or set visibility to hidden but this will be ugly)
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In Onchange make retrigerrable delay (It will execute after some time (disappear time) and if your health changes it will start counting again.)
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After delay finishes call this animation from point 3.