Update: The fix for this issue was released in the 3/12/2020 update:
Letter Log: The Letter Log page has been removed from the parent portal navigation. Instead, if parents are given Read permissions to the LTL (Letter Log) Table, they will see that information at the bottom of the Attendance page.
If using the District Settings option “Disable School Based Attendance Letters” AND also the Portal Groups for Parents and/or Students has Read permissions to Letter Log (LTL), the Letter Log page is not limited to their students.
Work-around: Remove the Read permissions to Portal Group permissions for Parents and/or Students to the Letter Log (LTL).
Programming is working on this now.