cliora,
Thanks for getting back in touch with us after reviewing those support articles, and for those additional details.
While some apps allow you to customize the Touch Bar, some don't. With that said, you might test the issue further by logging into a new user account to see if you're able to customize the Touch Bar in Preview there:
How to test an issue in another user account on your Mac
If it works in a new user account, try booting into safe mode in to your admin account to isolate incompatible items. Here's how:
Use safe mode to isolate issues with your Mac
If this concern isn't resolved, contact Apple Support for further assistance:
Get Support
Have a great day.