On the user-defined data list used in the custom email alert, make sure that the output fields contain a system date (Date Added or Date Changed). After adding that to the output fields go to the Set save options tab > Alert options sub tab > Mark the Allow this data list to be used for feed alerts checkbox > Set the system date field as the Publication date field > Save and close.

After making this change, add another record to the data list then run the custom email alert send process again.