I ran into something VERY similar with XCP-ng and\or XOA a while back - ended up with close to 200 new "clients" showing up on my Meraki gear. They weren't even "connected" long enough to register as IP clients, only MAC clients (ARP, possibly). I didn't think much about it at the time, as both the MAC addresses and hardware information didn't seem to be tied to any actual device by some manufacturer. I may be able to pull those out from my client history...

EDIT: Here is a small section of that list:
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