This is really a documentation (help text) issue, but I am classifying it as a broken function, because following the documentation literally causes the function to not work properly.
The help text indicates: "codes like %d, %M and %Y can be used in the backup destination filename. These will be replaced with the day, month and year"
However, according to the strftime man page, %M is used for minutes, and %m is used for the month:
%m The month as a decimal number (range 01 to 12).
%M The minute as a decimal number (range 00 to 59).
This error prevents the backups from working as expected. It appears on the Virtualmin backup pages at: