Still have a problem / don't understand what's going on.

camera.fog_start - distance from camera where fog is starting, yes?
camera.fog_end - place where fog is in "full" alpha, right?
and everything behind this point have fog color, right?

camera.clip_far - how far I can see, am I right?

What is the point?
- simplest level with white bg_color and couple of boxes or whatever in a row (from 200 to 2000)
- some model and 3rd camera
- black (1,1,1) fog from 500 to 1000 - everything farther than 1000 will be black but still visible
- so changing camera.clip_far to 1200 to make farther boxes unvisible
- everything looks ok, nice fog... only first boxes are visible
- then move your model behind camera.clip_far - and will disappear!
It's a bug?
HERE is a simplest level

I probably don't understand your problem, but objects disappearing behind clip_far is not a bug but the whole purpose of clip_far.

Yeah... but it's not refreshing like fog where distance from camera is always the same. Run the level.... if you set clip_far f.e. 2000 and you (player and camera) will be at 1800 you will not see models at 2100... just 300 away from you. And when you reach 2000 you will disappear.
Clip_far should be like a fog... always at the same distance from camera, am i right? But isn't.
Another words...
Fog it's always distance from camera.
Clip - from nullvector.

I don't think so.

Run demo and you will see

Your demo contains a bugged engine - 7.05 that was once uploaded for a beta test. That's why it doesn't work of course. Where have you purchased that version? Please urgently contact the support with your user info. I'm locking this thread until this issue is resolved.