Empty payload on unconfirmed packets

Hi there!
I’m using LMIC-arduino Library and I found out that every time the node sends an unconfirmed uplink ( for example after an ADR negotiation) the payload field is empty, so I’m loosing lora node’s data.
Anyone knows whether this is a LoRaWAN behaviour or is a library configuration?

I’d really apreciate any hint!
Thanks in advance,