This is to avoid the following behaviour: a user selects unitalicised text and updates footnote style to match all italics are cleared, including those that the user wants to keep, such as emphasised words.

⚠ Footnote style ignores bold, italics, underlining and linked text. For these to be picked up by Footnote Style, you must select an entire paragraph (this can be done by triple clicking the paragraph). ⚠ Attributes like text alignment apply to paragraphs, not text within paragraphs. Use the ‘Update footnotes’ command from the Footnote Style menu to re-apply your style to the document. New footnotes will revert to Google Docs’ default style. This will save your style settings and update all footnotes.

Then, with the footnote text highlighted, go to ‘Add-ons→Footnote Style→Match footnote style to selection’.

To set the footnote style you want, highlight an existing footnote or create one and style it in the way you want. Footnote style allows you to change the style of all of your Google Docs footnotes at once.

Add footnote in google docs