Meraki Get Network Events API - not returning clientMac

We have an org with 15 networks. We’re calling the following endpoint to pull some wireless event detail (client associations): https://developer.cisco.com/meraki/api-v1/get-network-events/ However, one a few of our networks (but not all), the clientMac is null on the returned data, whereas on most networks, its populated as documented. Before I raise with support, does anyone know of any reason why?