funny mysql issue i don't understand

Hi,
i'm running into a funny issue i can't understand with a simple query, this

Code:

$this->db->query("update table set field = NULL where id = xx");

doesn't do anything (whether the updated column is alone or aamong other cols, prepared query or not) and, of course, the field is set to be NULL by default
i must admit i do not understand at all why because pastnng exactly the same query in the phpmydamin sql tab query field it works like a charm!
(codeigniter 4.5.3)
if any of you had a lightful idea that would help me understand why my old brain is stuck
have a nice day

edited just because i forgot to say it's a date field