I have taken a bit of time to look into this, and what you are seeing appears to be leftover from how we used to handle tracking data. Constant Contact has recently moved most of our customers to a new contact management platform, and with the transition to that platform we have changed our systems for storing detailed email campaign tracking data so that it is kept for a number of years after the campaign is sent.
However, prior to this transition detailed data such as what URLs were clicked on by a specific user were only kept for 90 days from the time the email campaign was sent. When we transitioned users from the old system to the new system we gave them approximately 180 days of history back from the time of the transition and implemented the new policy to keep the data much longer.
Based on the information above, I find it very likely that the email campaign you are looking to get the tracking data from may have been sent before the new storage policy was in place and the detailed information may no longer be available. The best method to test this is to create a new test campaign with at least one link and send it to yourself. By employing this method, I was able to get the link ID and the link URL both in the API response.
If you do have any problems with trying this on a new campaign, please let me know!
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