We regret the inconvenience caused.
We have analyzed the reported query. As per our behavior, EJ1 Gantt has not allow predecessor to mapping with parent task. Hence, we have confirmed this as a bug and logged a bug report. You can also track the bug status through our feedback portal as below,
This fix will be included in our upcoming Volume 4, 2018 Service Pack 1 release which is expected to available on first week of February 2019.