This mostly happens when the Meraki devices are splitted in 2 networks OR you have Non-Meraki devices in the mix :
If Client A sends a ARP or any broadcast , it will reach both switches. Switch on the left 'may' sample the packets from client 'A' and update the client list with Client 'A's MAC on the uplink which 100% causes confusion because you lose track of where the client is actually coming from.
It is not by default because Meraki assumes that all devices are Meraki. The major drawback is that it is hidden and per-network and not org-wide. I'm pretty sure that you have to open a ticket to support everytime you create a new network from scratch ( a.k.a not cloned from that said network or from a template that already have that feature on ).
I'm not sure if someone from Meraki can affirm that or not , but this is what we were told this year.
Our distribution switches are Cat 6816 and this ''issue'' happens for clients that are seen on vlans that are spanned to multiple stacks on the same network. Most of our vlans are unique per stacks ( per floor ) so we don't often get that situation , but you know there is always an exception in every network 😒
EDIT : it is not often discussed but I found some post about it : https://community.meraki.com/t5/Dashboard-Administration/Clients-from-Other-Network-Showing-in-Dashb... https://community.meraki.com/t5/Switching/Clients-on-Uplinks/m-p/31989