There is a problem with the system notification in Workflow. You have a workflow with a several users. The usergroups of the users based on registered (no administrators), so they and should not receive the "normal" system messages.
If you set the option "Receive System Emails" to "No" in the user configuration the user get also no workflow notification, even if his user group and/or the user is explicitly in the configuration of the transition.
Set up a workflow scenario like. https://docs.joomla.org/Publishing_Workflow/Scenarios/en
But set receive email notification in the user configuration to "No":
As a result there should be emails sent to all members of the usergroup "faktencheck" and to the user "checker"
In the described configuration the selected usergroup and user get no email.
If you set the options "receive system messages" to "Yes" for all users which are involved the Workflow, each user get all messages from the workflow. Also from the transitions they were not selected.