Boolean Modifyer Difference shows different results at the same object

Hi there

I do have an issue with applying the Boolean Modifyer in “Difference”.
I wanted to cut out a hole out of the active object with a cylinder.
While it worked fine at the lower portion of the object, it showed a complete different result at the upper portion.
At the upper portion it didn’t cut out anything. So you couldnt see a result in Object Mode. While switching to Edit Mode, it now showed a round face within the Object but no cut. The round Face seemed to be connected with the face at the top of the actice object. Means if I selectes the round face and tried to alter it, I would automatically also alter the face at the top of the active object.

How comes, it is acting so differently although it is the same object and what would I need to do, for getting the same results also at the upper portion of the active object?

Thanks a lot