Why is option "At Angle to XC" not associative?

Hi,

I am trying to make a CSYS based on Absolute CSYS using option "Z-axis, X-axis, Origin". To set X-axis direction, I pick the option "At Angle to XC" and use previously defined expression (called XCangle).

The new CSYS is located propperly, but when I try to edit the expression afterwards it does not work. When I edit the new CSYS, it shows that X axis vector is fixed:

So the connection to the expression has been lost (or never established?).

Could somebody explain this unexpected behaviour? Maybe I am doing something wrong.

I know I can make it associatively with "Offset CSYS" option. I am just wondering why it does not work with "At Angle to XC" option.

Thanks in advance!