Unlike the constant of a similar name, this filter lives in PHP code. It can be added by a plugin, a must-use plugin or a theme, so it is easy to overlook when you only look at wp-config.php.
A management plugin, a hosting integration or a custom snippet may be using it deliberately to take over update control. It may also be a leftover from a developer who is no longer involved.
When updates are not arriving, check both the constant and the filter. Search plugin and must-use plugin folders for the filter name, and ask the host whether their platform applies its own update policy.