get_posts returns no results when run via wp-cron

I have a get_posts that I need to run regularly as part of a wp-cron triggered function – it works as expected if I call the function via a shortcode, but produces no results if triggered via wp-cron (though logging shows that the rest of the function is executed normally through to the end).

Can anyone think of a reason this should be the case?

// Setup arguments for topic query
$topic_args = array(
	'post_type'      => bbp_get_topic_post_type(), // Only bbPress topic type
	'posts_per_page' => -1, // All topics
	'meta_key'       => '_bbp_last_active_time',
	'fields'         => 'ids',
	'orderby'        => 'meta_value', // Order by _bbp_last_active_time (ie. from newest to oldest)
	'post_status'    => join( ',', array( bbp_get_public_status_id(), bbp_get_closed_status_id() ) ), // All public statuses
	'meta_query'     => array(
		array(
			'key' => '_bbp_last_active_time',
			'value' => date($current_time, $day_ago_time),
			'compare' => '>',
			'type' => 'DATETIME',
		)
	)
);

$topics = get_posts( $topic_args );

if ( $topics ) { // This code block is never executed if the function is run via cron