Cannot change the definition of AUTOMATIC_UPDATER_DISABLED

I have several WordPress sites that I want to enable auto update. I installed Background Update Tester to verify that the updates were enabled correctly, but I get the following message:

FAIL: The AUTOMATIC_UPDATER_DISABLED constant is defined as true.

It was defined as true in wp-config.php, but after I removed the line, it's still saying that the constant is true. I also tried to define the constant as false to no avail.

I also tried to add this filter inside a plugin:

add_filter('automatic_updater_disabled', '__return_false');

But WordPress still thinks the constant is true.

I ran a case insensitive grep for AUTOMATIC_UPDATER_DISABLED on my entire Wordpress directory with no results (except for class-wp-automatic-updater.php, which seems to check the definition).

What could be the problem?