Hi,
You could have maintained the output device for the purchase groups and test the scenario again!!!
In standard, the dispatch time is defaulted either from the default values from the output type - NACE settings or from the condition record in MN04.
Did you check whether there is any key combination with purchase group in MN04as explained in my previous comment? If there is no such key combination, then you may check whether any routine has assigned against the output type in the output procedure in NACE.
Regards,
AKPT