Assigned expression is evaluated twice when updating column TIMESTAMP NOT NULL

-- create table

drop table if exists foo;

create table foo(value TIMESTAMP NOT NULL);

-- insert one row

insert foo set value=now();

-- update of 1 row takes 2 seconds instead of 1 second

update foo set value=sleep(1);

-- insert one more row (two rows total)

insert foo set value=now();

-- update of 2 rows takes 4 seconds instead of 2 seconds

update foo set value=sleep(1);

-- ----------------------------------------

-- just for fun - make it NULL instead of NOT NULL:

alter table foo modify column value TIMESTAMP NULL;

-- update of 2 rows takes 2 seconds, as expected

update foo set value=sleep(1);