Closing shortcode showing onscreen

Things to try:
1. remove the extra parentheses:

<?php echo do_shortcode( '[expand title="Read More" class="bio-spoiler"]' . get_sub_field('staff_bio_cont') . '[/expand]' ); ?>

2. set your do_shortcode content to a variable:

<?php
$my_content = '[expand title="Read More" class="bio-spoiler"]' . get_sub_field('staff_bio_cont') . '[/expand]';
echo do_shortcode($my_content);
?>

3. Include a link to your page so we can be of further assistance.

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