BuddyPress.org

This problem comes from the bp_has_custom_signup_page function.

It checks to see if your theme can locate_template( array( 'register.php' ), false ) or locate_template( array( '/registration/register.php' ), false ) )

Since your child theme has neither of those files, naturally it will return false and give you wp-signup.php as the link. To counter this, BuddyPress attempts to be smart and redirect to BP_REGISTER_SLUG.

If you always want to bypass this check all together, you could try putting this in your bp-custom.php file…

function bp_custom_get_signup_page( $page ) {

global $bp;

return $bp->root_domain . '/' . BP_REGISTER_SLUG;

}

add_filter( 'bp_get_signup_page', 'bp_custom_get_signup_page' );

Note that bp_get_activation_page and bp_has_custom_activation_page work the same way, and the same rules apply if for some reason you want to bounce people from a different activation page.