Unexpected results using LEAST or GREATEST on aggregate

select least(min(action_date),cast('2000-01-01' as datetime)) from test; 

CREATE TABLE `test` (

  `action_date` date NOT NULL

) ENGINE=InfiniDB;

insert into test (action_date) values ('1999-01-01');

insert into test (action_date) values ('2020-01-01');

select least(min(action_date),cast('2000-01-01' as datetime)) from test;