The enterprise wireless I was connecting to required that your wifi mac address be known. By default, NetworkManager randomises your wifi mac address (https://blogs.gnome.org/thaller/2016/08/26/mac-address-spoofing-in-networkmanager-1-4-0/).
So to disable this just for this particular network, I run the following command (where SSID is the name of the wireless network):
nmcli connection modify SSID 802-11-wireless.cloned-mac-address preserve

As per the linked page, if you want to disable it globally, add the following to /etc/NetworkManager/NetworkManager.conf:

[device]
wifi.scan-rand-mac-address=no