This is probably due to LOD’s (Level of Details). This is actually a performance related feature of UE4 which helps to maintain a target frame rate by “simplifying” on the fly meshes and other scene elements, including terrains, when they are beyond a certain distance from the player.
If you open the Details panel of your landscape you will find some parameters related to that. For example LODDisstanceFactor. Try to play with that to have the terrain simplify or stay the same at different distances from the player.
You should definitely go through the official documentation on Landscapes where these aspects are explained: