Login does not work in combination with multiple registration

ho w are declaring the variable in your template?

Two things: you are calling the form but not rendering it and rather than confirming anonymity with isAnonymous() we should rather check to see that there is no authenticated user - with is()

Final code:

if (!\Drupal::currentUser()->id()) {
    $form = Drupal::formBuilder()->getForm(Drupal\user\Form\UserLoginForm::class) ; 
    $render = Drupal::service('renderer');
    $variables['login_form'] = $render->renderPlain($form);
  }

Oh guess mixed up the question

Yes it confirms uid but isAnonymous did not work for me as the form doesn't even get rendered. Try with my suggested code verbatim, logs in fine at my end.

Update
======
Do you have the registration forms from multiple_registration module and the login form on the same page?

Same issue here. I found a fix, although it's a bit clumsy and probably requires more testing.
The multiple_registration_form_alter implementation is a bit heavy-handed I feel - it should be more precise in the forms it's targeting. Having said that, my own 'fix' is just as heavy-handed. I haven't looked into the ramifications of the change I made, although my login block on my multi-registration pages now work.

Where the route is checked and the form altered based on that, I added another check.
Original ~line 205:

if ($route !== NULL && $route->getPath() === '/user/register/{rid}') {

I just added a check for the form_id too:

if ($route !== NULL && $route->getPath() === '/user/register/{rid}' && $form_id == 'user_register_form') {

There may be forms other than the registration form that need to be altered by multiple_registration_form_alter especially because there is a more specific alter later on in the code too: multiple_registration_form_user_register_form_alter.
Or it could be that the whole function could be merged into multiple_registration_form_user_register_form_alter.. either way, it works for me now.

Hopefully someone will get some use out of this patch. It applied cleanly for me on version 2.99.0 and fixes the issue

So the patch from @gumrol #9 does not apply to the latest release 3.1.1 as the line that #9 is changing is changed differently coming from
the issue https://www.drupal.org/project/multiple_registration/issues/3258961
I am hoping that this issue is now a duplicate and the other solution is good to go.

I updated to the patch from #9 to apply to the current 3.1.1 release. This resolves the issue for me.

Thank you @maddentim and @gumrol
for your effort and module support.

The #11 patch will be committed soon.