How to use strip_shortcodes() from a spawned process?

You can manually add shortcodes to the array of tags to be stripped in strip_shortcodes() via filter. If the global $shortcode_tags either doesn’t exist or doesn’t contain what it should when your process is running then you’ll need to use that filter to add them back in.

$tags_to_remove = apply_filters( 'strip_shortcodes_tagnames', $tags_to_remove, $content );