That doesn't seem right. You sure the mac address matches? If the ap was in the wrong vlan and or if the ap was not able to find the WLC, the ap would go through dhcp again. Maybe an issue with the native vlan or the vlan not being available, don't really know. I don't really look at the join stats unless I have issues, I look at the show ap summary and other commands for up to date info. That is just me though.
Yeah... I only run around 10 of them with no issues, but also utilize the RLAN on them. Mixture of 1815t and 1815w... I don't use static on them, basically all on dhcp.