Not intentional

The grammar support for ADD_MONTHS allows expr for both arguments, which results in an Item_decimal getting created for the second argument for rational numbers, in this case 2.6. When Func_handler_date_add_interval::add is invoked, it calls get_interval_value which relies on the item argument's val_int() implementation to do the right thing. In the case of Item_decimal, val_int() will return a value rounded to the nearest integer because it relies on my_decimal which in turn relies on my_decimal2int and decimal_round to handle rounding according to a policy flag. That policy flag is set to HALF_UP by default because my_decimal doesn't supply an alternative parameter to my_decimal2int. In the present bug, that causes 2.6 to be rounded to 3 which is returned by Item_decimal::val_int() during get_interval_value, thus resulting in 1994-10-07 instead of the expected 1994-09-07 as otherwise expected.

After a conversation with serg we decided to document the current behavior and close this as not-a-bug.

Updated documentation at https://mariadb.com/kb/en/add_months/