My understanding of SLAAC is if a client sees an RA packet and has IPv6 enabled it will assign itself an IP. You should be able to see what that is in a packet capture. It could be another device misconfigured on the network. That device would have to be reconfigured, taken offline, etc. The only reason I can see wireless clients only getting it would be if you bridge wireless to a separate VLAN than wired and the RA advertising device is only on that VLAN. I'm not aware and can't find any documentation that suggests IPv6 exists in NAT mode yet. (https://documentation.meraki.com/Wireless/Product_Information/Compatibility_and_Firmware/IPv6_Support_on_MR_Access_Points) Regardless the only way to completely prevent the clients using IPv6 is disabling it on the clients. Others may have better insight, we don't really use IPv6 in our environments so my experience is limited, but hopefully the above helps, but I do think the capture is the best bet, find the RA packet, get it's mac and hunt it down.
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