When using the field Registrations\Date Purchased or Mapped Registrations\Registartions\Date purchasted in Results fields to display of query, you may see that this field does not consistently populate for your Registrants.
We are currently evaluating this issue and will update this article when we have more information.
Steps to Duplicate
From Analysis, Click Information Library, Add an Ad-hoc Query
Choose the Source view of Registrants
On the left click Event, From the middle drag Event Record to Include records where and set equal to your event name
On the left click Registrations, From the middle drag Date Purchased to Results fields to display
On the left expand Mapped Registrations, click Registrations. From the middle drag Date Purchased to Results fields to display
Preview the query and see that Date Purchased is not populating for all constituents