Sessions no longer persisting when using array driver in Codeception tests

I have a new 5.1 project and set up the Codeception testing end of things pretty much the same as my last 5.0 project. I have a .env.testing file which codeception references and that has a SESSION_DRIVER set to array.

However for some reason now, sessions are not persisting.

For example if I do this in a functional test:

session(['test' => true]);
$I->submitForm('.test', ['title' => '']);
dd(session('test'));
$I->seeFormErrorMessages(['title' => 'The title field is required.']);

and run the test, the die and dump outputs 'null'.

If I remove the dd() then the expected formErrorMessage is non existent, but testing it manually in the browser the error message appears as expected.

Interestingly, if I tail the logs I can see that a TokenMismatchException is being thrown.

For interests sake I overrode the handle() method in App\Http\Middleware\VerifyCsrfToken to always return $next($request); and that correctly stops the TokenMismatchException being thrown, but the dd() still returns null, or with the dd() removed from above the test still fails.

If I change my SESSION_DRIVER in .env to file, then the test passes correctly with no errors, but if I try to run more than one test using the file driver, then the first test passes but after that codeception gets stuck on the second test, reporting nothing. I tried using database as a driver as well (I have the relevant session tables setup) and the same thing happened - runs fine with only one test, gets stuck on second test if there are more than one test.

.env.testing relevant variables

APP_ENV=testing
APP_DEBUG=true

DB_HOST=localhost
DB_DATABASE=l5_tests
DB_USERNAME=test
DB_PASSWORD=test
    
CACHE_DRIVER=array
SESSION_DRIVER=array
QUEUE_DRIVER=sync

Codeception version:

% vendor/bin/codecept
Codeception version 2.1.1

Codeception.yml

actor: Tester
paths:
    tests: tests
    log: tests/_output
    data: tests/_data
    helpers: tests/_support
settings:
    bootstrap: _bootstrap.php
    colors: true
    memory_limit: 1024M
modules:
    config:
        Db:
            dsn: 'mysql:host=localhost;dbname=l5_tests'
            user: 'test'
            password: 'test'
            dump: tests/_data/dump.sql

I'll do a clean install of L5 && Codeception in the morning and see if I can continue to replicate the issue, but thought I'd post this here in case anyone else is having the same issue.