sheet body convert to solid body when NX is closed

I want to extract the outer surface of a sphere (solid body), that is a linked body (named : Solid Body "BALL").

To do that, I used the "Extract Geometry" function, as shown on the picture below. 

A "sheet body" is created I added it to the reference set to see it clearly (named : Sheet Body "BALL" in the reference set). The violet surface in the picture.

So far, so good ... 

But the problem is appearing, when I close NX. Once I reopen the part, the previous sheet body is converted into a solid body !! 

It looks like it was converted as a solid, but only when NX has been closed.

Now, there are two solid bodies and no more sheet body in the reference set.

It's very odd, because it's a simple function...

I absolutely don't understand why a sheet body is converted automatically into a solid body ! 

Please let me know, if you have already this problem 

Cheers

Fabien