Fix handling of pre-prep tasks.

cylc/flow/scheduler.py

@@ -1245,7 +1245,7 @@ def release_queued_tasks(self) -> bool:
             # don't release queued tasks, finish processing preparing tasks
             pre_prep_tasks = [
                 itask for itask in self.pool.get_tasks()
-                if itask.state(TASK_STATUS_PREPARING)
+                if itask.waiting_on_job_prep
             ]
 
         # Return, if no tasks to submit.