So I found the error. At least the one in my case.
Since I am pretty new to this whole wordpress/html/css world it took a while.
The page where the shortcode [tooltip_glossary] was inserted was running the “SiteOrigin Editor“. I removed that widget in the Pagebuilder and replaced it with the standard html widget.
Now the collapse-o-matic [expand] shortcode does not show but is working as intended.
So obviously not any fault with your plugin. Maybe emphasize on your plugin installation tab that one needs to be sure that the code needs to be in a pure hmtl setting (for other newbies like me ?? )
Again, really appreciate the quick and detailed help. Since we will make a lot of use of tooltips on the site we will surely buy this plugin.
Have a nice day!
Sam