problem in pppoe connection when changing the pool IPs

hi to all…

I have a DMA radius manager with Mikrotik X86 pc & about 1600 pppoe users …

I have a pools like this 11.12.0.1-11.12.0.254, 11.12.1.1-11.12.1.254…etc

but when trying to change the pools to 172.16.0.1-172.16.0.254, 172.16.1.1-172.16.1.254…etc some users ( about 100 to 150 users only ) connect with 0.0.0.0 ip so they can’t use internet
I kick them out but they reconnect withe the same ip 0.0.0.0

then I reconfigured the pools to old ips & every thing is ok !!!

any idea about that ?