We are seeing the same thing on a client site.
The bar isn't showing on the pages it's supposed to
Not to mention The WP Notification Bars plugin is bloating their error logs Here is a small snippet > https://prnt.sc/26t9nxm
Here is what all of the error logs are saying
PHP Warning: Undefined array key "HTTP_REFERER" in /home/########/public_html/wp-content/plugins/wp-notification-bars/includes/class-wp-notification-bars-shared.php on line 558
What can be done to fix this issue?
This is not a conflict issue, we have checked and the error persists regardless of what plugins or themes are active...
Cheers,
Ash