i like to post an update to my post above, which makes it more complicated...

above I said activity BB is a future activity but marked as 100% by giving future actual start and finish dates. (although this can not happen, i know, but my schedule was not updated for few weeks and i knew now that a few days ago that activity started and then finished, so i marked it that way)

now i updated my data date. and now activity BB dates are past.

but still the same thing happens. when i make activty B a predecessor of activity AA, and enter actual start and finish dates which are different to AA, duration of 0 is calculated to AA by the program