MQTT - Downlinks

Having migrated my application and devices to V3. My MQTT downlinks which were working on V3 have stopped working and return
When I look on the V3 console:
The downlink message has been received OK with the payload intact, but is dropped because of an “Unknown session” error
“data”: {
@type”: “”,
“namespace”: “pkg/networkserver”,
“name”: “unknown_session”,
“message_format”: “unknown session”,
“correlation_id”: “1b14362dff7d4f54a8d093a87a9d0764”,
“code”: 5

Any ideas?

Edit: I solved the problem by creating a new application and migrating the devices. It works fine now.

I did get the exact same error message yesterday as wel, not by scheduling a downlink by MQTT but within the console.