That is how mobility works in an lwapp environment. Remove the wlc's on the mobility group and the user will drop and have to reassociate. No need to have the wlc's in the mobility if you want to have different dhcp scope.
What you just simulated was inter controller roaming. WLC v4 and above support auto anchor, which means whenever a client move to another AP on a different controller (in this case, same AP but move to different controller), the connection will be anchored to their original controler (in your case WLC2), thus it will keep the IP address on this site. So nothing wrong with this at all. repairing or reconnectiong the client will get them back to WLC1.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
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