Objects disappear at the wrong distance

I created a runway boundary with Blender, similar to cones you already know. Quite simple. However, the parts do not disappear at the correct distance.
I loaded cones from someone else’s existing set and they behaved without any problems. So I did a test with 2 cubes that are inside an invisible cube (one single object).The LOD value is of course shown for the largest invisible cube. The LOD value in the statistics profiler is listed as 0.5%. The small cube disappears at approx. 10.0 and the large one at approx. 0.9. If the value falls below 0.5, the displayed value disappears from the monitor. Both disappear in such a way that they are seen as clearly faded out.

Shouldn’t all parts of the object disappear at the same time?
What am I doing wrong?
The cone from the other package also works correctly, so it can’t be the settings of the FS. The cone is also inside an invisible cube with another large visible surface (under the ground) and both disappear at the same time. The FS also shows only one LOD value for the 3 cubes including the invisible one.

Thanks for help.
All the best Ralf