Technician Auto-Assign vs. Business Rule
If you have selected the option All requests under Tech Auto-assign then even if the Business rule sets a technician, tech auto-assign will reset the technician value.
However, when Tech Auto-assign is set as an Unassigned request, Technician Auto-assign will get applied after the Business rule. Since the Business rule has not set the technician for a request, tech auto-assign will set the technician for that request.