As explained above, a new version was introduced a while ago. With the new version, the descent path as computed by the FMGS takes priority in order to make all the altitude constraints (as here in the US) and the speed is allowed to drift away. Vertical restrictions and descent path has priority over speed, this is called latched descent. The VDEV has a small magenta box along the altitude display to simbolize the vertical path is now latched or has priority.
Actually in real world ops you can see this mode and works beautifully, now one has to solve the issue of ATC asking to maintain a lower speed as well. For this we are gonna use the speedbrake for example.
The traditional method had it that speed has priority over the vertical constraints. If a constraint was predicted to be missed due to a shallower descent path, than crew had to use speedbrake while the engines were at IDLE and DES mode was active. If the crew forgot/not deployed the speedbrakes, at some point the PFD and FMGS wold pop up a more drag message to warn the crew to use whatever they could to save the vertical profile with that selected speed.