yes, you’re using the same field name in both the outer and nested row. See the comment I added to your code
<div>
<!-- Check for parent repeater row -->
<?php if( have_rows('menu_sections') ): ?>
<ul class="nav nav-tabs" role="tablist">
<?php $row = 1; // number rows ?>
<?php // Step 1: Loop through rows, first displaying tab titles
while( have_rows('menu_sections') ): the_row();
// ?>
<li role="presentation" class="list-unstyled<?php if($row == 1) {echo 'active';}?>">
<a
href="#<?php echo $row; ?>"
role="tab"
data-toggle="tab"
>
<?php the_sub_field('section_title'); ?>
</a>
</li>
</ul>
/***************************************************************
The field name used in the outer loop should be menu_sections just
like the above loop. You may also need to reset_rows() here.
***************************************************************/
<?php if( have_rows('section_items') ): ?>
<div class="tab-content">
<?php $row = 1; // number rows ?>
<?php // Step 2: Loop through rows, now displaying tab contents
while( have_rows('section_items') ): the_row(); ?>
<?php // Display each item as a list ?>
<div class="tab-pane <?php if($row == 1) {echo 'active';}?>" id="<?php echo $row; ?>">
<ul>
<li class="list-unstyled"><?php the_sub_field('dish_name'); ?></li>
<li class="list-unstyled"><?php the_sub_field('dish_description'); ?></li>
<li class="list-unstyled"><?php the_sub_field('dish_price'); ?>
</ul>
</div>
<?php $row++; endwhile; // (have_rows('section_items') ):?>
</div>
<?php endif; // (have_rows('section_items') ): ?>
<?php $row++; endwhile; // (have_rows('menu_sections') ):?>
<?php endif; // (have_rows('menu_sections') ): ?>
</div>
