Update query in AFTER INSERT is ignored
I'm not sure why your trigger isn't working - unless it's because other rows have been inserted in between the two triggers firing.

I suspect your issues may be mitigated by using a compound trigger along the lines of:

CREATE OR REPLACE TRIGGER stg_control_inserts
  FOR insert on Distributionplan
    COMPOUND TRIGGER

    v_job_id stg_control.job_id%TYPE;
    v_job_name stg_control.job_name%TYPE := 'DistributionPlan';

   BEFORE EACH ROW IS
   BEGIN
     INSERT INTO stg_control (job_name, job_id, execution_start, exec_tocore_start)
     VALUES (v_job_name || ' ' || SYSTIMESTAMP, v_job_id, SYSTIMESTAMP, SYSTIMESTAMP)
     RETURNING job_id INTO v_job_id;
   END BEFORE EACH ROW;

   AFTER EACH ROW IS
   BEGIN
     UPDATE stg_control
     SET    execution_end = '14/05/19 00:00:00,493000000'
     WHERE  job_id = v_job_id;
   END AFTER EACH ROW;
END stg_control_inserts;
/


N.B. untested.