Spawn Undermap when moving to otherside of terrain

This is the first time i have used a terrain, i made it the biggest it could be but every time i either use spawn from camera location on play, or just walk to a certain point, i get teleported underground at the, i guess start of the map, somewhere in the middle.
Does anyone know what’s causing this or how i can fix it.

EDIT: when i walk to this certain point, my character just disappears and i am unable to move, and if i use the spawn at camera location i then get teleported to the center of a BSP brush (see below) and my character also disappears /

Also there is what appears to be a BSP platform in my level that i cant move, select, delete or anything and its clipping through my ground when i get up high, i also have a feeling this might be causing the sudden death i get when walking a certain distance.

And another thing keeps happening upon saving, sometimes i get an error saying it cannot be saved and its actually deleting my level save and i have to re create the save?

Disabling “Enable world bound checks” in world settings fixed this problem