It turned out the problem was specific to one particular laptop that had recently been repaired at the Apple Store.
The laptop had been joined to Meraki MDM prior to going in for repair. The main logic board was replaced at the Apple Store, and although the serial number stayed the same it must have changed some other hardware identifier. After coming back from repair, the laptop was reformatted but was unable to join Meraki MDM.
The solution was to Delete (Remove From Network) the laptop from the list of Devices in Meraki Systems Manager. After that, I had no trouble re-joining the laptop into Meraki MDM. This was a surprise, because normally it's not necessary to delete device entries, as they automatically merge based on serial number.