You could force any URL that contains a `preview_id` URL parameter to the WordPress front-controller (ie. `index.php`). This will effectively bypass the WP code block for these type of URLs. (It is the WP code block that _skips_ these requests when they map to your static directories.)

(This avoids having to modify the existing "WordPress" rules, as I mentioned in comments.)

Add the following _before_ the WordPress code block, ie. _before_ the `# BEGIN WordPress` comment marker.

    # Send any URL with "?preview_id=" immediately to the WP front-controller
    RewriteCond %{QUERY_STRING} ^preview_id=
    RewriteRule ^ index.php [L]

    # BEGIN WordPress
    : Remaining directives follow...