User authenticating but not staying logged in Laravel 8.x - laravel

Question

When testing my application and trying to create a new user then on form submission I am getting error 419.
If I use a helper to log in using Auth::loginUsingId(x) for an existing user then dd(auth()->user()) is successful but when I redirect to another page the user is no longer logged in. eg running the following code, you end up getting redirected to /login:
Route::get('/logInAs/{id}', function ($id) {
Auth::loginUsingId((int) $id);
dd(\auth()->user());
return redirect()->route('dashboard');
});
I would share more code but I'm not sure what would be useful at this point?
I've probably spent about seven hours trying to fix this so far. It's driving me up the wall!
The app works fine on other environments, such as staging. I've even gone so far as to re-download it from github as a fresh install (in case I was messing about in vendor directory and changed something), but that hasn't fixed it.
I get the same issue on Chrome, using different signed in users on Chrome, using incognito mode on Chrome, using Firefox, and using valet share and accessing my local version on another device, like a phone, so I don't think it is to do with browser caching.
When other devs working on the project run it locally on their machines I haven't had reports of this issue, so it seems to just me my local environment.
I've variously tried changing CACHE_DRIVER and SESSION_DRIVER in .env to no joy, but I'm beginning to lose the will to live so don't really know if I'm doing the right thing there - please help if you can!!
Edit to add more detail:
Middleware on the dashboard route:
Route::middleware(['auth:sanctum', 'verified'])->group(function(){
Route::get('/dashboard', DashboardController::class)->name('dashboard');
});
If I remove the middleware then I just get an error that user is null on dashboard.blade.php, ie the user is still not staying logged in.
A custom middleware:
class VerifyUserIsMerchant
{
public function handle(Request $request, Closure $next)
{
if ($request->user()->isMerchant()) {
return $next($request);
}
throw new AccessDeniedHttpException('You must be a merchant to access this page');
}
}
If I dd($request->user()) in this middleware it returns null.

Answer 1

The issue got fixed with the help of Laracasts. The full answer is here.
In short, I needed to remove the http:// protocol from SESSION_DOMAIN in .env and set it to match APP_URL.
This was the correct answer for this issue. It was deleted by a moderator. Moderator, before you delete this answer as well please can you explain why?
It provides the full answer to the issue raised in the question. I did not find this answer to this issue anywhere else (hence why it took me several days of debug to fix).
If I can know why it was deleted, I can then understand how to avoid making answers in the future that also would be deleted. Thank you.