This caused trouble in the Astro sites which source content from WordPress sites like this.

The 'Select posts' page is a block page containing selectors for

• By date — WP Archives block. Works in Astro.
• By category — WP Categories list. Doesn't work in Astro.
• By author — ncvp_author_list shortcode because WP doesn't provide a select by author block. Works in Astro.
• By tag — WP Terms list. Doesn't work in Astro.

The problem was that in Astro selecting by category or tag loaded site-url/?category_name=xxx or ?tag=xxx, which Astro can't deal with.
It turned out that the best solution is to completely replace the WP /select-posts page with select-posts.astro, which is similar to the page on Astro sites with their own content collections.