It looks to me that you are coping FORMATTED text and then paste the text. In this case LibreOffice will just paste FORMATTED text and override your settings. This works as designed, so user that already has formatted text will not lose formatting after pasting…
One of the way to solve this problem is to paste non-formatted text, but this can’t be done without some advance clipboard manager. I have installed Ditto clipboard manager on Windows from http://ditto-cp.sourceforge.net/
Start Ditto and select Options from right menu.
Keyboard Shortcuts
Text only paste: type in letter T and check Win checkbox. (you can set any other shortcut if you like)
Now copy text like ordinary CTRL+C and when you would like to paste formatted text, just paste with CTRL+V, but when you would like to paste non-formatted text (like in your case), then press Windows_key+T and text gets pasted non-formatted in this case the font you have selected before pasting is going to be used.
One other thing. You wrote that you select courier 12 before pasting, maybe you also need to change template to have this font and font size to be selected automatically using paragraph style…