is likely due to the LOD switching that occurs when you use engine scalability settings lower than high. Go to Settings in the toolbar -> Engine Scalabilty -> set it to Epic and try again.
is a cut & paste from my answer to the first linked thread below, detailing what I think is causing :
See further here as well:
https://forums.unrealengine/showthread.php?48460-3rd-person-character-going-through-terrain-in-some-places
https://forums.unrealengine/showthread.php?34815-Dropping-Beneath-Landscape