This often happens when there's an MX attached to the "core" switch in a Meraki network, and the reason is due to the MX not properly handling LLDP/CDP.
https://documentation.meraki.com/MX/Networks_and_Routing/MX_Layer_2_Functionality#Spanning_Tree_Prot...
I linked to the STP section of that document, but the LLDP/CDP section below also has similar behaviour. The MX will forward LLDP/CDP out every port other than the port it is received on, which violates those protocol guidelines. With regards to the topology diagrams this has the side effect of making the dashboard think there's another device in the network when there actually isn't.