I think this is the same as what I've found in the Email Campaign Management starter solution.
Do the tags look like this: <Last Name> when you insert them?
If so, you may have a global text field in your table, In the Email Campaign Mangement solution, the field is called g_body_select. Open Manage | Database | Fields and see if you can find this field. If you can find it double click it and see what is entered as an auto-entered calculation. If it's the same as this starter solution, you'll find a substitute function that replaces each tag with data from a field in your solution.
You'll need to debug this calculation in order to figure out why data from the middle name field is being entered in place of the Last Name tag.
That's it! Thanks so much for the quick reply.