When I open the "File" menu or any of the other menus in LibreOffice or GNU Image Manipulation Program, why do my xfce4-keyboard-settings shortcuts stop working until I click off the menu in Ubuntu Studio 18.04 running the Xfce version 4.12 Desktop? Thanks for your time everyone.
This is not a problem. Just close your menu before using your global shortcut keys.
When you press a key, your operating system will evaluate it. The desktop environment, or a window manager, or any other hotkey daemon will get to look at the shortcut first. If the key matches a global shortcut, e.g. for launching your terminal, the action corresponding with that global shortcut key will be executed. In other words, globally defined shortcuts will take precedence over shortcut keys defined at the application level. This behavior allows you to swithc windows, manipulate windows, and have quick launchers available (almost) any time. If no global action corresponds with the key you pressed, control is passed to your current application, that can do something with it if actions are defined for that hotkey.
Indeed, while the application menu is open, globally defined shortcut keys are not being evaluated anymore. That is a design decision of the developers. While the menu is open, you have no option than to do something with it before you can do something else. This actually makes sure that all your application menus will be closed at times where you do not need it.
If you ask me, this helps reducing confusion and keep things neater. I do not find a need to come back to an application one hour later, just to see that some menu is still halfway open.