Xbee S1 sending strange messages

Hi, I am running a test over 2 Xbee S1 modules: the sender sends a broadcast frame every 5 seconds and receiver simply receives and displays the data. Both modules are set in API mode and MAC is “802.15.4 w/acks”. Node Discover NO is disabled. However, the receiver has some strange behavior. Every time it has received a frame, it sends some strange 802.15.4 frames back to the broadcaster, and the broadcaster replies. I can also see this from a 802.15.4 sniffer.
Can anybody explain what is going on? I also attach the Sniffer output. I can also provide Python source code.

Update: if I changed MAC to be “802.15.4+MaxStream header”, The receiver will not display any strange receives, but the sniffer still shows some extra frames exchanged between the broadcaster and the receiver.

What firmware version are you running on your EOL’ed XBee S1 modules?

Product family: XB24, Function set: XBee 802.15.4, Firmware version: 10ef
Thanks.