Sometimes there will be no gap that spawns between the charging horses, which means no matter what you fail to dodge the charge and the boss despawns. Pretty sure that is not meant to be how it works.
Steps to reproduce:
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Summon the Dark Rider boss fight.
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Observe sometimes, when the horse charge mechanics happens (at 75%, 50%, and 25%) the horses will have no gap between them, and cannot be dodged, thus resulting in a despawned boss.