Julūsan is a cognate/absolute object (mafʿūl muṭlaq), not a regular direct object that would make jalasa transitive.
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Do Not Count the Maṣdar as Direct Object
Intransitive and Transitive Verbs: One, Two, Three Objects
Julūsan is a cognate/absolute object (mafʿūl muṭlaq), not a regular direct object that would make jalasa transitive.