The existing font is retained when "Default" font is chosen for selected text in note body.
I can confirm this.
Here's what I did:
1) I copy/pasted from WordPerfect into L4 notes.
2) I noticed that the font from WP was retained.
3) To get it to revert to the default font I selected the 'offending' text and selected "Default" as the font.
4) Then I changed the focus to another note.
5) When I return to the original note, I find that the font reverts to the previous setting.