Add_rewrite_endpoint doesn’t work with post name permalink structure

I would avoid using add_rewrite_endpoint() directly to register account page endpoints in WooCommerce. WooCommerce has a filter that you can use that make sure everything else works properly, like the endpoint title filter and calls to is_wc_endpoint_url().

So add the endpoint with the woocommerce_get_query_vars filter:

add_filter(
    'woocommerce_get_query_vars',
    function( $query_vars ) {
        $query_vars['conversations'] = 'conversations';

        return $query_vars;
    }
);

Note that unlike the query_vars filter, you need to use a key when adding to the array.

Now you can set the page contents with the woocommerce_account_conversations_endpoint hook:

add_action(
    'woocommerce_account_conversations_endpoint',
    function() {
        // Page content here.
    }
);

And the title with the woocommerce_endpoint_conversations_title filter:

add_filter(
    'woocommerce_endpoint_conversations_title',
    function( $title ) {
        $title="Conversations";

        return $title;
    }
);

And you can continue adding it to the account menu the way you are.