CoAP Notification acknowledge failure

[TomazCebekAstron]

Hello,

we have a situation, we keep receiving repeats of observed topic notifications.
Running emqx 4.3.4...

We receive a notification (hex format):
44 45 0a ae 00 11 22 33 61 01
61 2a ff 7b 22 35 30 22 3a 22
which is generaly translated into:
{
"type": "Confirmable",
"code": "Content",
"id": 2734,
"token": 1122867,
"options": [
"Observe(value=1)",
"Content-Format: application/octet-stream"
]
"payload":"7B 22 35 30 22 3A 22"
}
We acknowledge this message with (hex format):
64 44 0a ae 00 11 22 33
which is generaly translated into:
{
"type": "Acknowledgement",
"code": "Changed",
"id": 2734,
"token": 1122867,
"options": [
]
}

As you can see we acknowledge the notification...
Why does the coap plugin keep sending the same notification?

Do we need to configure libcoap differently? Or does this plugin not work anymore?