Cloned MAC address entry fails validation for non-hexadecimal values
Setting the cloned address for a given connection profile to the value "random" by issuing
nmcli will randomize the MAC upon each connect.
nmcli c modify ef3e8535-f8b5-45bb-a732-02a7a142b5c6 802-11-wireless.cloned-mac-address random
Editing the connection profile afterwards with network-manager-applet will not allow you to change any setting values as the cloned address text field seems to fail validation.
libnm-core/nm-setting-wireless.c and libnm-core/nm-setting-wired.c says: Beside explicitly specifying a MAC address, the special values "preserve", "permanent", "random" and "stable" are supported.