Out parameter is sometimes null/blank without any error, need to find root cause

[hieunm89]

I'm using DS 3.1.9
I have a python task that simply get the current datetime and store in an out parameter. Most of the time it works well, however sometimes that parameter is blank and downstream task cannot read it, without any error in the upstream task. I've check var_pool in db and confirm it was blank. Found nothing unusual in worker log.
Is the anyway to check the root cause?
Task script:

from datetime import datetime
now = datetime.now()
start_process_time = now.strftime("%Y%m%d%H%M%S")
print('${setValue(start_process_time=%s)}' % start_process_time)

var_pool value of this task instance in database:

[{"prop":"start_process_time","direct":"OUT","type":"VARCHAR","value":""}]