I found that part confusing as well, along with the diagram below that paragraph. That section seems to first indicate that a constraint will be missed if the FCU ALT is not increased in time, which makes sense. Then it states that all constraints between the aircraft actual altitude and FCU ALT are deleted, which makes less sense as the aircraft has presumably already leveled off at the FCU ALT.
But then again, on Page 7, it shows a typical climb profile with the FCU ALT at T/C and the aircraft leveled off at the altitude constraint below it, which is consistent with paragraph I highlighted earlier.