Changing Font back after copying and pasting item
It tends to happen because the last space or 'new line' didn't get deleted too, so any further typing is already set to that new font. This can happen in any text app, it's not Word-specific.

The 'proper' way to fix it is to make sure everything was tidily deleted &amp; that the font from the end of the current part is as it should be...
...which is a complete nuisance to bother with, so...

The 'clever' way to do it is just go back to before the last full stop [.] (period US Eng) &amp; start typing from there instead.
That way you know that the font, at that point, was already set to what you were using.
You can forward delete that full stop if you wish, it tends to prevent auto-correct &amp; spellcheck from working properly whilst it's there.

An alternative, but only if you haven't already spent some time doing specific formatting, is to select all  Cmd ⌘   A  then re-select your chosen font, which will set the entire document.