Foreach循环适用于font-page.php但不适用于page.php？

I made a simple custom meta box plugin to display testimonials. I used a foreach loop to retrieve and display the data on my front-page.php page. However, using the exact same code on my page.php page returns

Warning: Invalid argument supplied for foreach().

What is the reason for this error and how can I fix it?

Here is the code used on both front-page.php and page.php:

     <div id="testimonials">
            <?php 
            $testimonials = get_post_meta($post->ID, "testimonials_data", true);
            $counter = 0;
            foreach($testimonials as $testimonial){
            $counter++;
            ?>
            <div class="testimonial <?php if($counter == 1){echo 'active-testimonial';}; ?>">
                <p><?php echo $testimonial['field1']; ?></p>
                <span><?php if(!empty($testimonial['field2'])){echo $testimonial['field2'];}; if(!empty($testimonial['field3'])){echo ' - ' . $testimonial['field3'];}; ?></span>
            </div>
            <?php }; ?>
        </div>

EDIT: To be more clear. My problem isn't that the foreach loop doesn't work at all. It works on the front-page.php but not page.php using the same code. How do I get it to work on both pages?

Your get_post_meta() $single attribute is set to true. Try setting it to false so that it always returns an array.

$testimonials = get_post_meta($post->ID, "testimonials_data", false);

get_post_meta( int $post_id, string $key = '', bool $single = false )

This function return (mixed) values - It Will be an array if $single is false. - It Will be value of meta data field if $single is true.

so if you have stored value in string testimonials_data except json_encoded. then it will show you value in array only.

and apply some !empty check

 <div id="testimonials">
        <?php 
        $testimonials = get_post_meta($post->ID, "testimonials_data", true);
        $counter = 0;
        if(!empty($testimonials)){
            foreach($testimonials as $testimonial){
            $counter++;
        ?>
        <div class="testimonial <?php if($counter == 1){echo 'active-testimonial';}; ?>">
            <p><?php echo $testimonial['field1']; ?></p>
            <span><?php if(!empty($testimonial['field2'])){echo $testimonial['field2'];}; if(!empty($testimonial['field3'])){echo ' - ' . $testimonial['field3'];}; ?></span>
        </div>
        <?php }; 
        }?>
</div>