Shared calendar is not writeable for certain user

Nextcloud version: 12.0.3
Operating system and version: Fedora
Apache version: Apache/2.4.27
PHP version (eg, 5.6): PHP 7.1.9 (fpm-fcgi)
Is this the first time you’ve seen this error?: yes

Can you reliably replicate it? (If so, please outline steps):
I have a system set-up with the following accounts:
Nils
test1
test2

When I create calendars with “Nils”, share them with others and give write access, other users see them as an abonnement instead of an editable calendar.

Also when I create a calendar with user “test1” and share it to “test2” and “Nils”, user “test” is able to edit the calendar and “Nils” is not. Eventhough both have write access.

Screenshots: https://imgur.com/a/NdrYQ

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'instanceid' => 'xxxxxxxxxxxxxxxxxx',
  'passwordsalt' => 'xxxxxxxxxxxxx',
  'secret' => 'xxxxxxxxxxxxxxxxx',
  'trusted_domains' => 
  array (
    0 => 'xxxxxxxxxxxxxx',
  ),
  'datadirectory' => '/var/www/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '12.0.3.3',
  'dbname' => 'xxxxxxxx',
  'dbhost' => 'xxxxxxxxxxx',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'xxxxxxxxxxxxx',
  'dbpassword' => 'xxxxxxxxxxx',
  'installed' => true,
  'forcessl' => true,
  'maintenance' => false,
  'htaccess.RewriteBase' => '/',
  'overwrite.cli.url' => 'https://cloud.mydomain.com/',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'updater.secret' => 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxx',
  'theme' => '',
  'loglevel' => 2,
  'knowledgebaseenabled' => false,
);