Word doesn't renumber the footnotes because the footnote is still there—albeit marked for deletion. When the changes are accepted or rejected, then Word will renumber the footnotes accordingly.

How do I automatically update footnote numbers in Word?

When you want to automatically update the supra reference numbers, do this: Click your cursor anywhere inside the footnote fields. Hit Ctrl A. Hit F9.

Do footnotes automatically reorder themselves?

Endnotes appear at the end of the page on which they are inserted. Footnotes and endnotes automatically reorder themselves as the user enters or deletes other footnotes and endnotes. In order to move a citation, the user must cut the original and paste it in the new location.