In all of the below cases permalinks have been flushed. The endpoint works perfectly fine for all scenarios _except_ the home page.

rewrites-1.php

Using only this, hiting ^/bpa/whatever/ serves blog archive instead of static front page.

<?php

/**
 * Create url endpoint for BPA
 *
 * @since  1.0.0
 */
function cfpurl_rewrite_rules() {
	add_rewrite_tag( '%bpa%', '([^&]+)' );
	add_rewrite_endpoint( 'bpa', EP_ALL );
}
add_action( 'init', 'cfpurl_rewrite_rules' );

rewrites-2.php

Using only this, hitting ^/bpa/whatever/ redirects to ^/

<?php

/**
 * Create url endpoint for BPA
 *
 * @since  1.0.0
 */
function cfpurl_rewrite_rules() {
	add_rewrite_tag( '%bpa%', '([^&]+)' );
	add_rewrite_endpoint( 'bpa', EP_ALL );
	
	$homepage = get_option( 'page_on_front' );
	add_rewrite_rule( '^bpa(/(.*))?/?', 'index.php?p=' . $homepage . '&bpa=$matches[2]', 'top' );
}

rewrites-3.php

Using only this, hitting ^/bpa/whatever/ redirects to ^/

<?php

/**
 * Create url endpoint for BPA
 *
 * @since  1.0.0
 */
function cfpurl_rewrite_rules() {
	add_rewrite_tag( '%bpa%', '([^&]+)' );
	add_rewrite_endpoint( 'bpa', EP_ALL );
}
add_action( 'init', 'cfpurl_rewrite_rules' );


function cfpurl_home_query_update() {
	if ( is_admin() ) {
		return;
	}

    global $wp_query, $wp_the_query;

    remove_action( 'parse_query', 'cfpurl_home_query_update', 11 );

    // If we have a homepage and should be showing it, we have to
    // completely reset the query and make sure it's handles as
    // the "main" query, for it to recognize this properly without
    // a redirect.
    if ( 0 === strpos( $_SERVER['REQUEST_URI'], '/bpa/' ) ) {
        $homepage = get_option( 'page_on_front' );

        if ( $homepage ) {
            $wp_query = $wp_the_query = new WP_Query( array(
                'bpa' => get_query_var( 'bpa' ),
                'page_id' => $homepage,
            ) );
        }
    }
}
add_action( 'parse_query', 'cfpurl_home_query_update', 11 );

Do you know anything about if this was fixed? I still have this problem...