My search results are showing all indexed nodes regardless of the search text.

Also, this error is logged:
User error: "0" is an invalid render array key in element_children() (line 7457 of /srv/www/core/includes/common.inc).

In this line

element_children() is called with an array containing a key->value where the value is the search string. element_children() throws an error when the value is not an array.

If I bypass the is_array() in https://github.com/backdrop/backdrop/blob/96a0e03870c9fd09ab4134827a05148173e8e516/core/includes/common.inc#L7448 , the error isn't thrown and the search results are correct.