"...Curl, ACK over the air is sent after the packet was received and verified for that module by the Zigbee Stack...."
Eric, thank you for helping. Based on your response (if I undertanding it), the ACK is sent as long as the packet is verified on module, but BEFORE transmitted to application by UART. So, could the input buffer be corrupted by a new incoming packet (the application cannot have time to recover the entire packet).