When the same nav menu is shown multiple times on a page, only the first instance of the nav menu will include `id` attributes. Subsequent instances will have the `id` attributes omitted. However, in both cases the underlying nav menu item's post ID is available among the `class` names, so the post ID can be more reliably obtained from `class` instead of the `id` attribute.
Amends [36383].
Fixes#32681.
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