Automatic re-train is a "feature" you cannot turn off, cannot turn on, have no visual feedback that it occurs, and ultimately weakens the security of your device.
If this is what is causing Face Unlock to fail then that's still just as much Apple's responsibility than any other methodology. You also have no way to knowing how much re-train has occurred since Apple doesn't provide that information.
Automatic re-train is a "feature" you cannot turn off, cannot turn on, have no visual feedback that it occurs, and ultimately weakens the security of your device.
If this is what is causing Face Unlock to fail then that's still just as much Apple's responsibility than any other methodology. You also have no way to knowing how much re-train has occurred since Apple doesn't provide that information.