Aleksander - that is the way INTERFAC works. When going from cut to fill, the 'flip' point is interpolated at the level datum string (LDS), giving the situation on the left of your diagram.
You can specify a barrier string (261/262 - field 1=BARR) to get this point moved away from the LDS, but you will still have to manually amend the strings - see Example 26 in the Help file -Linemode>Interface.
You can specify a barrier string (261/262 - field 1=BARR) to get this point moved away from the LDS, but you will still have to manually amend the strings - see Example 26 in the Help file -Linemode>Interface.