An anonymous user must be identified by session ID (line 176 -> line 223)

@jcmartinez I also work on an Open Social project and I have found one place, where the aforementioned patch could be applied, it's in social_group module in GroupMuteNotify.php file at line 54, so the fix there should theoretically look the same like in message_subscribe module:

$flaggings = $this->flagService->getAllEntityFlaggings($group, $account, \Drupal::request()->getSession()->getId());

I will try to reproduce the error and then test this fix, but if you get to it sooner then let us know please how it went. :)

Pretty sure the message subscribe module causes a lot of related issues in other modules.

For Open Social it was indeed related to group mute function, so custom code and not the Flag module itself.
https://www.drupal.org/project/social/issues/3355315 →

Is it still relevant?

I'm not entirely sure after the latest updates(MS) but it's still relevant when using the message subscription module.