Error on multisite: health check – wp_version_check() – WordPress 5.6

After updating a wordpress multisite to version 5.6 I see a critical security warning on the health check page of all subsites except the main site:

A plugin has prevented updates by disabling wp_version_check().

I searched where the error message originates and found that it is generated when the function test_wp_version_check_attached cannot find a filter for the function wp_version_check.

So I checked where this filter might be set and I found the following lines in wp-includes/update.php close to the end of the file:

if ( ( ! is_main_site() && ! is_network_admin() ) || wp_doing_ajax() ) {
  return;
}

add_action( 'admin_init', '_maybe_update_core' );
add_action( 'wp_version_check', 'wp_version_check' );

If I understand these lines correctly, there is a return statement before add_action( 'wp_version_check', 'wp_version_check' ); can be executed, when either is_main_site() or is_network_admin() is false. And since is_main_site() is false for all of the subsites’ health check pages, this return statement will always kick in for those.

For testing purposes, I tried the following:

if ( ( ! is_main_site() && ! is_network_admin() ) || wp_doing_ajax() ) {
  add_action( 'wp_version_check', 'wp_version_check' );
  return;
}

add_action( 'admin_init', '_maybe_update_core' );

and the error message on the health check page of all subsites disappeared.

This looks like a bug to me.

In case it is not, I am thankful for all and any hints of how to get rid of the error.