researchers found that "the overwhelming majority of
  Android devices are not implementing the available
  randomization capabilities built into the Android OS,"
  which makes such Android devices trivial to track.
  [...]
  Apple, meanwhile, introduced MAC address randomization in
  iOS 8, only to break it in iOS 10. [...] its network probe
  broadcasts to include a distinct Information Element (IE),
  data added to Wi-Fi management frames to extend the Wi-Fi
  protocol.

  four scholars from the US Naval Academy say they've
  managed to track 100% of all test smartphones, despite
  the devices using randomized MAC addresses. 
  [...]
  The novelty in our method is that we are sending RTS
  frames to IEEE 802.11 client devices, not APs, to
  extract a CTS response message which we derive the true
  global MAC address of that device.