Canonical tag missing for AMP pages

After updating to v4.2.4, AMP pages no longer have a rel canonical tag.
It must be caused by the following code:

if ( ! empty( $canonical ) && ! aioseo()->helpers->isAmpPage( 'amp' ) ) :
	?>
		<link rel="canonical" href="<?php echo esc_url( $canonical ); ?>" />

But, without this tag, Google complains about such AMP pages:

Referenced AMP URL is self-canonical AMP
AMP pages with this issue are invalid. Invalid AMP pages aren’t served on Google

What was the point of removing the canonical tag and how can it be fixed?