Hi Ivan,
Thanks for contacting Syncfusion support.
We have checked your reported query with shared sample but we could not replicate the issue "Gantt chart cannot render deep child task when the property CollapseAllParentTasks = true at first render" at our end. Also, we have prepared the video demo for reference.
Please check with shared video and we request to share more issue replication steps to proceed further.
Regards,
Alagumeena.K