Linux print jobs stay <defunct> until application terminates

[António]

Dear group,

I have some Linux workstations sending print jobs to networked printers. These are shared via Samba as raw queues.
Server and workstations are Centos 7, all using  CUPS 1.6.3.

Jobs are small (labels printed on demand, one by one) and get printed quickly and good, but the respective processes stay defunct at the workstation until the Harbour application is terminated:

 2908 tty1     S+     1:24 bin/producaol
 2909 tty1     Z+     0:00 [lpr] <defunct>
 2911 tty1     Z+     0:00 [lpr] <defunct>
 2915 tty1     Z+     0:00 [lpr] <defunct>
 2917 tty1     Z+     0:00 [lpr] <defunct>
 2919 tty1     Z+     0:00 [lpr] <defunct>
 2921 tty1     Z+     0:00 [lpr] <defunct>
 2924 tty1     Z+     0:00 [lpr] <defunct>
 2926 tty1     Z+     0:00 [lpr] <defunct>
 2928 tty1     Z+     0:00 [lpr] <defunct>
 2930 tty1     Z+     0:00 [lpr] <defunct>
 2932 tty1     Z+     0:00 [lpr] <defunct>
 ... ....
 3011 tty1     Z+     0:00 [lpr] <defunct>
 3013 tty1     Z+     0:00 [lpr] <defunct>
 3015 tty1     Z+     0:00 [lpr] <defunct>

The print commands are:

sSetDevice := SET(_SET_DEVICE, "PRINTER")

@ prow(), pcol() say cEtiq

set(_SET_DEVICE, sSetDevice)
SET PRINTER TO

man ps says:
  «Processes marked <defunct> are dead processes (so-called "zombies")
  that remain because their parent has not destroyed them properly. These
  processes will be destroyed by init(8) if the parent process exits. »

Am I doing something obviously wrong that leaves these processes alive ?
Any advice will be apprecitaed, Thank you
António Vila-Chã
Harbour 3.2.0dev (r1607041701)