FAQPage schema in PHP not formatted correctly - json

Question

I'm trying to build a schema for a page that has FAQ questions. The code works but something is wrong with the formatting. The problem is that the array "mainEntity" is on every question.
Here is my code
$posts = get_field('section_faq_relation');
if( $posts ):
$schema_faq_questions = array();
foreach( $posts as $post):
setup_postdata($post);
$headline = get_the_title();
$articlebody = get_the_content();
$schema_faq_question = array(
'#type' => 'Question',
'name' => $headline,
'acceptedAnswer' => array(
'#type' => 'Answer',
'text' => wp_filter_nohtml_kses( $articlebody )
)
);
array_push($schema_faq_questions, $schema_faq_question);
endforeach;
$schema_faq_all = array(
'#context' => 'http://schema.org',
'#type' => 'FAQPage',
'mainEntity' => $schema_faq_questions
);
echo '<script type="application/ld+json">' . json_encode($schema_faq_all) . '</script>';
endif;
wp_reset_postdata();
My code looks OK and returns this if I look at the json it's outputting. mainEntity is an array that holds all the questions.
{
"#context": "http://schema.org",
"#type": "FAQPage",
"mainEntity": [
{
"#type": "Question",
"name": "dolor sit amet",
"acceptedAnswer": {
"#type": "Answer",
"text": "Lorem ipsum dolor sit amet, consectetur adipiscing eli"
}
},
{
"#type": "Question",
"name": "quasi architecto",
"acceptedAnswer": {
"#type": "Answer",
"text": "re veritatis et quasi architecto beatae vitae dicta sunt explicabo."
}
}
]
}
But if I run the schema.org validator it returns:
Every question has "mainEntity". I don't know why it's doing that.