We also had the same issue, the Workorder completion date and start date were including Non-working days.
"Why Discrete Jobs Form Is Not Computing Completion Date Based On Workday Calendar? [ID 1450120.1]"
The above metalink note might help you. All the resource on the assembly must be "Available 24 hrs" and make sure you have not inculded Saturdays and Sundays as a working day in you Manufacturing calendar (Assigned at org parameter).