Update - This was fixed in the update released on 9/21/2018
Known Issue - When deleting a Letter Log record for Attendance Letters, the Student Attendance fields (STU.LT and/or STU.L2) are not decremented. This issue occurs as of the 8/17/2018 update.
Work around - To ensure the correct letters are generating for the student with the deleted Letter Log records, manually decrease the applicable student Attendance fields (STU.LT - Absent and/or STU.L2 - Tardy) on the Attendance page.
Programming is working on this now and the fix should ship in the next update.