If a paragraph format is defined without "font style" selected in the Manage Formats window, then selecting that format, whether affecting only selected text or the entire paragraph, would be expected to leave font style unchanged (bold remains bold, italics remains italics). Present behavior is that the font style is reset to whatever the format's font style is for all affected characters.
In the attached file, place the cursor in any paragraph that has bolded text. Select either body or body-serif pp format. The bold text will change to plain text.