I tested several different scenarios with WP Permalinks and WP internal rewriting and I see the same thing where only the main site URL is redirecting to other site’s main URL using this code: RedirectMatch 301 ^/$ http://example.com/$1
. So creating individual additional redirect rules would be the best method to ensure that all old URL’s are redirected to the new URL’s.
RedirectMatch 301 ^/$ http://example.com/$1 RedirectMatch 301 ^/post-name-1/$ http://example.com/post-name-1/ RedirectMatch 301 ^/post-name-2/$ http://example.com/post-name-2/