In stable docs, search icon (magnifying glass) in top nav doesn't display

Resolved for latest docs by #737 but stable docs are still impacted.

The search button in the top nav bar displays with only a keyboard shortcut hint, and no icon:

https://docs.jupyter.org/en/stable/contributing/content-contributor.html

I'm using Firefox 133.0. Also tested in Chromium 119 and the behavior is the same.

When I click the space where the search icon should be, the icon flashes into existence for a fraction of a second and disappears again!