Salesforce: Field History Tracking

By design, when a field is enabled for tracking, it is not tracked during record creation. For example: Primary Campaign Source (PCS) in Opportunity:

Action	Appears in Field History?
Create Opp with PCS filled	❌ No
Create Opp blank PCS	❌ No
Update PCS from blank → Campaign	✅ Yes
Change PCS Campaign A → Campaign B	✅ Yes

The same result using SOQL, sample: 

SELECT Id, OpportunityId, CreatedDate, Field, DataType, OldValue, NewValue FROM OpportunityFieldHistory WHERE DataType = 'Text' ORDER BY CreatedDate DESC