The content disappears when saving a custom field.

Hi,

You can try to create a staging site and disable all plugins except Meta Box, MB extensions, switch to the standard theme of WordPress and re-check this issue.
Let me know if the issue still happens.

Also, my custom fields were imported from ACF. Could this be causing a problem?

Here is the PHP code generated for one of the WYSIWYG fields:

<?php
// Getting the value:
$value = rwmb_meta( 'intro' );
echo $value;
?>

<?php // Displaying the value: ?>
<h2>Content</h2>
<?php rwmb_the_value( 'intro' ); ?>

<?php
// Parse shortcodes and add paragraphs:
$value = rwmb_meta( 'intro' );
echo do_shortcode( wpautop( $value ) );
?>

Here is a maps field

<?php
// Displaying the value:
rwmb_the_value( 'local_map' );
?>

<?php
// Getting the value:
$value = rwmb_meta( 'local_map' );
echo $value;
?>