Microsoft Teams send Activity Feed Notification through Graph API - azure

Question

I have a with yeoman teams scaffolded tab in which I ask for adminconsent for application permission.
On a Go backend I then use the code to retrieve the access token. I am also getting all Users of the team.
Although I am using only application permissions, I can not send notifications to the admin account. When trying to send notifications to other accounts, the Graph API returns a 204 Status Code but the activity feed of the recipient stays empty.
The application has following application permissions (with granted admin consent):
ChannelMember.ReadAll
GroupMember.ReadAll
Team.ReadBasic.All
TeamMember.Read.All
TeamsActivity.Send
This is the Manifest file with version 1.11:
{
"manifestVersion": "1.11",
"id": "{{APPLICATION_ID}}",
"version": "{{VERSION}}",
"permissions": [
"identity",
"messageTeamMembers"
],
"webApplicationInfo": {
"id": "{{APPLICATION_ID}}",
"resource": "api://{{DOMAIN}}.{{APPLICATION_ID}}"
},
"devicePermissions": [
"notifications"
],
"activities": {
"activityTypes": [
{
"type": "newActivity",
"description": "A new Activity Description"
}
]
}
}
The HTTP POST request to: https://graph.microsoft.com/v1.0/teams/{teamID}/sendNotification
The JSON Body:
{
"topic": {
"source": "entityUrl",
"value": "https://graph.microsoft.com/v1.0/teams/%s"
},
"activityType": "newActivity",
"previewText": {
"content": "A new Activity Description"
},
"recipient": {
"odata.type": "microsoft.graph.aadUserNotificationRecipient",
"userId": "{{USER_ID}}"
}
}
The recipient is also Member of the channel the activity is referring to. I also made sure that the recipient can receive notifications.