In Nougat and earlier, ambient display won't kick on if the proximity sensor is covered. For example, if the phone is in your pocket. Android O takes that a step further and will actually turn the display off again if the sensor is covered. Again, imagine putting the phone back in your pocket after checking ambient display. It's easy to brush the screen in ambient mode and wake the phone up completely.
But, in Android 8.1 beta preview turn off this feature. However, that's less likely to happen if the screen shuts off as soon as the proximity sensor is covered. mobile devices won't go to sleep covering proximity sensor its kind of annoying.
It's possible this will change in the final build, or it could simply vary between devices. Still, it's something to keep an eye on.