HOUR() function returns NULL when it should not

CREATE TABLE test_stats1 (

  ts DATETIME DEFAULT NULL

) ENGINE = Columnstore;

CREATE TABLE test_stats2 (

  ts DATETIME DEFAULT NULL

) ENGINE = Aria;

INSERT INTO test_stats1 VALUES ('2017-07-07 12:00:00');

INSERT INTO test_stats2 VALUES ('2017-07-07 12:00:00');

SELECT HOUR(TIMEDIFF(MAX(ts), now())) FROM test_stats1;

-> NULL

SELECT HOUR(TIMEDIFF(MAX(ts), now())) FROM test_stats2;

-> 73

SELECT TIMEDIFF(MAX(ts), now()) FROM test_stats1;

-> -74:43:31

SELECT TIMEDIFF(MAX(ts), now()) FROM test_stats2;

-> -74:43:34