What you're seeing in the dashboard is expected behaviour.
The MAC Address seen in the dashboard is a physical identifier for the device.
The MAC addresses of the uplink interfaces are different from this, typically offset by 4 or 5.
The following doc describes this behaviour
Calculating MX WAN MAC Addresses - Cisco Meraki
As a side note, this behaviour looks to have changed on newer MX models.
I have an MX75 in which the dashboard MAC address matches the MAC showing in the connected switch's ARP table.