Illegal offset type in isset or empty when rendering flag link for User as anonymous user

Hello, I've investigated this issue and I think that the flag module is working correctly here.

Code snippet that is added to description seems incorrect in how parameters are passed to build method:
$link = \Drupal::service('flag.link_builder')->build($flag, $user, \Drupal::currentUser());

If you check the interface of FlagLinkBuilder:
1st parameter should be entity_type_id, which in this case should be 'user'.
2nd parameter needs to be user id for which this link is being built.
3rd parameter should be id of the flag you want to built link for, in your case you can use 'follow_blogger' or $flag->id()
4th parameter would need to be $view_mode in which you want this rendered in.

Can OP please verify whether he is passing parameters correctly or try using something like
$link = \Drupal::service('flag.link_builder')->build('user', \Drupal::currentUser()->id(), $flag->id(), 'default');