When that works, it means some erroneous computation was going on.
If it doesn't work, then it means that FMGC 1 and 2 positions are significantly different, which can happen in long flights in non GPS fitted models. You can compare the miles to destination in the bottom F-PLN page to check the extent of the difference. Even for a slight difference the non AP coupled FD bar will appear too displaced in DES mode.