table of contents bold italic
Use a style for a header that converts text to bold face.
Use that header style to have header text that includes some italics.
In the Table of Contents pane, associate that header style with a style that is Regular type face.
When updating the Table of Contents, the entry for that header will correctly have Bold text replaced with Regular text, but will leave Bold Italic text unchanged. It should replace Bold Italic text with Regular Italic text.