proxy_fcgi error sending session cookie using wp_remore_get - php

Question

Background
I have an application that has a Wordpress plugin. I'm posting it here because AFAIK wp_remote_get() is basically a wrapper around CURL, and I don't think this is a Wordpress problem. The application (call it the back end) has a REST API. Most of the requests to the back end happen via ajax from javascript in the browser, and both Wordpress and the back end application have access to the same session - this is tested and working - the plugin has access to all the same session data that the back end does.
The problem
For some tasks the plugin needs to make requests to the back end directly (as in not ajax from the browser), so I need to pass the session cookie with those requests which are made via wp_remote_get(). Here is the code:
$headers = array(
'Content-Type' => 'application/json',
'Api-Key' => $options['api_key']
);
if ( !empty( $_SESSION[ 'Security' ][ 'accessToken' ] ) )
{
$headers[ 'Authorization' ] = 'Bearer ' . $_SESSION[ 'Security' ][ 'accessToken' ];
}
$cookies = [];
$cookie = new WP_Http_Cookie( 'main' );
$cookie->name = 'main';
$cookie->value = $_COOKIE['main'];
$cookie->expires = time()+120;
$cookie->path = '/';
$cookie->domain = '.example.com';
$cookie->samesite = 'none';
$cookie->secure = true;
$cookie->httponly = true;
$cookies[] = $cookie;
$request = wp_remote_get( $api_url, array(
'headers' => $headers,
'cookies' => $cookies
)
);
Now, I'm not 100% sure that this will give the back end access to the session as expected, because when I send this cookie along with the request I get an error:
[proxy_fcgi:error] [pid 1737121:tid 140694932940544] [client 192.168.1.1:40819] AH01071: Got error 'PHP message: https://example.com/shop/api/v3/item?id=100101
PHP message: fmldcke15n4uco3622i2t0nlj1
fmldcke15n4uco3622i2t0nlj1 is the current session_id(), which I find odd.
I have played with the various settings for the cookie, and the values being set are copied directly from session_set_cookie_params() from the back end.
So, 2 questions:
Why would sending a session cookie like this cause a hard error in fcgi?
Assuming I can get this to work, will this allow the back end to access the values in session when called in this fashion?