Unable to deploy multiple eventSubscription resources; first one always succeeds, but the subsequent ones fail with "Conflict" error

We are trying to deploy Microsoft.EventGrid/systemTopics resource, which contains several Microsoft.EventGrid/systemTopics/EventSubscriptions resources.

We have no problem deploying the the system topic resource, as well as ONE subscription; however, the subsequent subscriptions always fail with "Conflict" error. We have tried both serial and parallel deployment - it fails with serial as well.

Attached please see the screenshots of the error itself from the portal. The error itself looks like that:

{
"status": "Failed",
"error": {
"code": "Publisher Notification Error",
"message": "Failed to enable publisher notifications.",
"details": [
{
"code": "Publisher Provider Error",
"message": "PUT request for https://management.azure.com/subscriptions/[subscriptionid]/resourceGroups/xio-bicep-resg/providers/microsoft.devices/iothubs/[hubname]/eventGridFilters/event-subscription?api-version=2018-01-15-preview failed with status code: Conflict, code: and message: ."
}
]
}
}