Zigbee sensor Mac ID changed to FF

Hi,
I am using xbee s2c modules,After i observed data from zigbee sensors to the coordinator the mac id of sensor changed to FF.This issue occurs after power off and on again the coordinator module.How mac id of devices changes?

Below i added the data from sensors.

Explicit RX Indicator (API 1)

7E 00 19 91 FF FF FF FF FF FF FF FF 49 2F 01 01 00 06 01 04 00 18 25 0A 00 00 10 00 9A

Start delimiter: 7E
Length: 00 19 (25)
Frame type: 91 (Explicit RX Indicator)
64-bit source address: FF FF FF FF FF FF FF FF
16-bit source address: 49 2F
Source endpoint: 01
Destination endpoint: 01
Cluster ID: 00 06
Profile ID: 01 04
Receive options: 00
RF data: 18 25 0A 00 00 10 00
Checksum: 9A