I had this problem on most of the MikroTik devices... I have 4 MikroTiks in my LAN. One od them is router and has assigned static IP and configured DHCP offering static IPs for other devices. From time to time, other MikroTiks change their MAC addresses of their bridge ports. As the result, they get different IP from DHCP. Usually the difference is like they used to have had XX:XX:XX:XX:XX:16 and after some time I can see it's XX:XX:XX:XX:XX:17... Anybody knows what is the reason?