When streaming enabled is turned on, Terrain begins to do what appears to be viewport culling when a player’s camera looks away from a segment of terrain. Doesn’t happen to every part of the terrain, only at certain sporadic segments.
https://gyazo.com/ad96f5a7b5239e4ff61d42b900b0bc12.mp4
Streaming Enabled settings are as follows: 
How repeatable is this problem for you? Does it happen every time you join the game? Does turning off streaming enabled eliminate the problem entirely?
This doesn’t happen every single time in game, but it’s common enough to have been noticed in multiple play sessions so far today.
The testing was done online in an active place, as opposed to studio.
Turning off streaming enabled removes the problem entirely.
Some bonus info for you:
When this issue occurs, it always seems to occur with this other issue where terrain in the distance that already has loaded in is rendering with a low mesh LOD, when it shouldn’t be rendering so low (causes the terrain to be blocky in shape).
These following images were all in sequence to one another.
If you’d like an RBXL of the terrain, let me know.
Also just for extra node, when the terrain is being culled like that last image above, its physics are still rendering, so I can walk on it without falling through the ground.
Also I was running this on Level 10 graphics.
Something you may want to try to do is increase the Y axis of your camera. Due to grass interactions you may be tripping off built in systems for keeping your character from being blocked.
It looks like when it happens the camera is being moved to an area with a vertical incline
