Webhook not triggerd by MS Graph Group Conversation

Question

In a customer szenario I'm working on a solution where I want to respond on incoming emails from a M365 group mailbox. I've set up a webhook to subscribe to the group mailbox and trigger an Azure Function.

The Azure Function is written in PowerShell:

using namespace System.Net

# Input bindings are passed in via param block.
param($Request, $TriggerMetadata)

# Write to the Azure Functions log stream.
Write-Host "PowerShell HTTP trigger function processed a request: $($Request.Query)"

# Interact with query parameters or the body of the request.
$validationToken = $Request.Query.ValidationToken

if ($validationToken) {
    Write-Host "Function was called with Validation-Token: $($validationToken). Returning Token"
    Push-OutputBinding -Name Response -Value ([HttpResponseContext]@{
        StatusCode = [HttpStatusCode]::OK
        Body = $validationToken
    })
    return
}else{
     Write-Host "Validation-Token not found"
}

$notifcations = $Request.Body.value

Write-Host "Got $($notifcations.Count) notifications"

# Associate values to output bindings by calling 'Push-OutputBinding'.
Push-OutputBinding -Name Response -Value ([HttpResponseContext]@{
    StatusCode = [HttpStatusCode]::OK
    Body = "Webhook called successfully"
})

The webhook registration is done via a POST request using Graph Explorer. URL: https://graph.microsoft.com/v1.0/subscriptions Body:

{
    "changeType": "created,updated",
    "clientState": "Subscription Test",
    "expirationDateTime": "2024-04-17T04:26:28.7712196Z",
    "notificationUrl": "{function-url}",
    "resource": "groups/{group-id}/conversations"
}

In the log console of the Azure Function, I can see that the validation request reaches the Azure Function. However, when I send an email to the group mailbox, the Azure Function is not triggered.

I've already replicated this behavior in another tenant.

When I don't register the webhook on the group conversations but instead, for example, on the group owner, I receive information in the Azure Function when a user is added to the group owners, for instance.

Unfortunately, the Microsoft Support wasn't helpful either.

Can someone replicate the behavior?