Nice the say make your own mesh terrain. I did long ago.
But mesh terrain have a big problem with how high fidelity is loading entities.
The mesh does not load if the center is behind you. Only strange thing is that the mountain is loading fine.
Moving your camera 45 left or right does not help. You really need to face more then 90 degree in this case to the center of the prim before the terrain load.
And the road entitiy with some grass is not really big.
Or we need soon normal terrain, or interface need to load more entities.
other example: a big spaceship hull would not load in this case to i would guess.