OK, I am backin the office, so I have done some test about this visualization.
1. open NX
2. file->utilities->customer defaults
3. select Gateway->visualization->view/screen tab. There you have a setting 'Maximum size for an Object's Bounding Box'. In my NX version, this is set to 265000mm
4. I have set this value to just 100mm and clikc OK on customer defaults dialogbox
5. restart NX
6. now, create a simple Block with Dimensions 200x200x200. After some zooming, most likely you will not see the whole body.
7. Click Fit command, and it will be OK.
8. now change the Box to some extreme values like 10000x10000x10000.
9. you can click Fit or any other command as you want, and you will not see the whole body
10. Now, change the customer defaults settings back to the original value. In my case, 265000mm
11. Restart NX and open again the box, you have created before. The one with 10000x10000x10000 values.
12. click Fit and you will see the whole box again.
So, Customer Defaults setting Maximum size for an Object's Bounding Box is the one, that controls, what will you see. It is like a space around your model.
And as I described before already, the problem is with rotating and zooming. When you watching the screen and moving the cursor, you have only two dimension: up/down and left/right. So you have no idea, have far behind (or infront) your model you will click. And because of that, when you rotate about some virtual points and zoomin/out a lot, it can happen, that the part of the model will disappear.