Work around for https://github.com/WordPress/gutenberg/issues/4989

block.js

(function (wp) {
    //WordPress APIs
    //Render function
    var el = wp.element.createElement;
    //Translations
    var __ = wp.i18n.__;
    //Text input control
    var TextControl = wp.blocks.InspectorControls.TextControl;
    //Create block
    wp.blocks.registerBlockType('shelob9/metatest2', {
        title: __('Meta 2 and Markup', 'metatest'),
        category: 'common',
        edit: function (props) {
            //Make sure "fake_content" and "content" are in sync
            if( props.attributes.fake_content !== props.attributes.content ){
                props.setAttributes({fake_content: props.attributes.content });

            }
            //Edit interface
            return el(
                'div',
                {

                },
                [
                    el(
                        'p',
                        {
                            className: props.className
                        },
                        props.attributes.content
                    ),
                    el(
                        TextControl,
                        {
                            label: __('Content', 'metatest'),
                            value: props.attributes.fake_content,
                            onChange: function (value) {
                                //Update both attributes
                                props.setAttributes({
                                    fake_content: value,
                                    content: value
                                });
                            }
                        },
                    )
                ]
            )

        },
        save: function (props) {
            return el(
                'p',
                {className: props.className},
                //Save the non-meta attribute
                props.attributes.fake_content
            );
        }
    });
})(
    window.wp
);

block.php

<?php

function metatest_enqueue_block_editor_assets() {
	$dir = dirname( __FILE__ );
	$block_js = 'metatest/block.js';
	$editor_css = 'metatest/editor.css';
	wp_enqueue_script( 'metatest-block', plugins_url( $block_js, __FILE__ ), array(
		'wp-blocks',
		'wp-i18n',
		'wp-element',
	), filemtime( "$dir/$block_js" ) );
	wp_enqueue_style( 'metatest-block', plugins_url( $editor_css, __FILE__ ), array(
		'wp-blocks',
	), filemtime( "$dir/$editor_css" ) );
}
add_action( 'enqueue_block_editor_assets', 'metatest_enqueue_block_editor_assets' );


add_action( 'init', function(){
    register_block_type( 'shelob9/metatest2', array(
        'attributes'      => array(
            //Meta attribute to use for post meta
            'content'      => array(
                'type' => 'string',
                'source' => 'meta',
                'meta' =>  'metatest_content'
            ),
            //Non-meta attribute to use for post content
            'fake_content'      => array(
                'type' => 'string',
                'source' => 'html',
                'selector' =>  'p'
            ),

        )
    ));
    register_meta( 'post', 'metatest_content', array(
        'show_in_rest' => true
    ));
});