Role of the downlink formatter in V3

I did not find much documentation and am still not clear about the role and correct usage of the downlink formatters.

From the console, I can use the downlink facility manually to send some data to a node. Usage is very similar to V2. But when I try to get my downlinks via MQTT i do not get it working.

So, what is happening if no downlink formatter ist set? will the byte message still be forwarded or is the message ignored? Are there any changes to get the downlink via MQTT set? BR