My guess is..
FMS will change phase as long as you input into FCU with either..
1. AP + FD On (normal lazy).
2. AP Off, FD On (hands on, using FD).
3. AP On, FD Off (no thrust mode, only speed mode)
With both AP + FD Off, how can FMS predict anything
. Hence no change over and no hockey sticks. I would prefer removing A/Th as well in such a situation. If reqd re-engage A/Th once you commence approach.
Will check the next time around.
Ah one more thing, in such a situation, might as well get into App Phase as early in descend, coz Des Phase serves no purpose.