The error is present in the WPP.
The Cloudflare API shows that the value 0 is equivalent to selecting “Respect Existing Headers”.
Read about here: https://api.cloudflare.com/#zone-settings-change-browser-cache-ttl-setting
In /includes/addons/cloudflare/views/admin.php
<?php
$options = [
30, 60, 300, 1200, 1800, 3600, 7200, 10800, 14400, 18000, 28800, 43200, 57600, 72000, 86400, 172800, 259200, 345600, 432000, 691200, 1382400, 2073600, 2678400, 5356800, 16070400, 31536000
];
?>
<tr>
<td>
<strong><?php _e( 'Browser Cache Expiration', 'wpp' ); ?></strong></td>
<td>
<select name="cf_browser_expire" form="wpp-settings">
<option value=""><?php _e( 'Respect Existing Headers', 'wpp' ); ?></option>
<?php foreach( $options as $option ): ?>
<option value="<?php echo $option; ?>" <?php wpp_selected( 'cf_browser_expire', $option, 14400 ); ?>>
<?php echo $option; ?>
</option>
<?php endforeach; ?>
</select>
There is no defined value for Respect Existing Headers
This solution seems to work well.
<?php
$options = [
0, 30, 60, 300, 1200, 1800, 3600, 7200, 10800, 14400, 18000, 28800, 43200, 57600, 72000, 86400, 172800, 259200, 345600, 432000, 691200, 1382400, 2073600, 2678400, 5356800, 16070400, 31536000
];
?>
<tr>
<td>
<strong><?php _e( 'Browser Cache Expiration', 'wpp' ); ?></strong></td>
<td>
<select name="cf_browser_expire" form="wpp-settings">
<?php foreach( $options as $option ): ?>
<option value="<?php echo $option; ?>" <?php wpp_selected( 'cf_browser_expire', $option, 0 ); ?>>
<?php if ($option == 0): ?>
<?php _e( 'Respect Existing Headers', 'wpp' ); ?>
<?php else: ?>
<?php echo $option; ?>
<?php endif; ?>
</option>
<?php endforeach; ?>
</select>
