Zend certified PHP/Magento developer

How can I “debug” my Magento website to find out why some text is there/not there?

first, a picture with some explanations. Then some questions…

Description of the behaviour of my website

The question is, can I somehow find out why the Text “Ab” is missing in some cases (when I initially load the website).

With the developer tools in the browser I found that it is triggered by some CSS?
But I have no clue what to do from there. I want to change this behaviour, that the initial state is “show ‘Ab’ near the price”.

Developer tools view from the browser

Does anybody know how I can “debug” this on the server?
What are the steps to find out more?

If you want to take a look at the web page on our staging server (if links are not allowed, tell me and I remove it): https://staging.relax-pillow.de/set-stillkissen-maxi-mit-bezug-grosse-210-x-44-cm.html