Thanks! There seems to be an ACF-related function on that hook page that looks like it might work so I will test that out.
function wpdocs_edit_canonical_urls( $original_url, $post ) {
$override_url = get_post_meta( $post->ID, 'canonical_url', true );
if ( empty( $override_url ) ) {
return $original_url;
}
return $override_url;
}
add_filter( 'get_canonical_url', 'wpdocs_edit_canonical_urls', 10, 2 );