Input:
<span title="<?php get_the_last_modified_timestamp()); ?>">
Last modified: <?php echo get_the_last_modified_timestamp('single-post', array('datef' => 'Y-m-d', 'format' => '%date%')); ?>
</span>
Expected:
<span title="2019-02-02@12:32">
Last modified: 2019-02-02
</span>
Actual:
<span title="<span class="last-modified-timestamp">2019-02-02@12:32</span>">
Last modified: <span class="last-modified-timestamp">2019-02-02</span>
</span>
which is invalid HTML.
Try this
<span title="<?php echo wp_strip_all_tags(get_the_last_modified_timestamp()); ?>">