When the Project task is closed complete the successor project task would go to the open state.
OOTB business rule 'Process State Change' has customized
As per the design it is intended to update the successor activity to open state on closure of the Predecessor project task. Please find the below OOTB scripts that are responsible for this behavior.
Business rule:Process State Change where on state change it is calling the script include 'PlannedTaskStateChangeHandler'
Refer line:100, where on change of the active flag it is calling another script include 'PlannedTaskUtilsV2;' by passing 'PlannedTaskStateUtil.OPEN_STATES' as a parameter to the function 'startSuccessors'
2)PlannedTaskUtilsV2- This is the script include that updates the state field of the successor project task. Please refer line 83
3)PlannedTaskStateUtil- Script include where the open state constants are defined.