Hi,
Thank you for the details, it helps a lot!
The issue caused by a 3rd party plugin “JS Help Desk”. It loads it’s stylesheets on our plugin back-end as well, and adding additional CSS rules, which then cause the problem: https://i.imgur.com/9cnr9SZ.png
That file should not be loaded there, or the CSS rule should be more specific so it does not override unwanted elements. Disabling the plugin even temporarily will resolve the issue.