Looks like problem is in End_Node sample application. In file lora.c function:
bool LORA_send(lora_AppData_t* AppData, LoraConfirm_t IsTxConfirmed)
doing a check of AppData buffer (actually size of this buffer!!!)
if( LoRaMacQueryTxPossible( **AppData->BuffSize**, &txInfo ) != LORAMAC_STATUS_OK )
And instead of send my payload go to send an empty frame:
{
// Send empty frame in order to flush MAC commands
…
And such behavior I found not only in B-L072Z_LRWAN dev board, but also in Dragino LoRa I/O Controller LT-33222-L. I think Dragino device used the same STM32 library inside.
Will go deep to research why an empty frame was sent each minute except first time after OTAA Join. Maybe something is related to ADR settings between LoraServer and devices.