Hi. I do it manually until the bug is patched.
I add many waypoints all along the route to guess the flight level thought by the simulator.
I set, if necessary, the constraint altitudes of origin and destination in the MCDU F-PLN.
I set the maximum flight level I found in MCDU F-PLN as cruise altitude.
Then I calculate the top of descente like this: (FL CRZ - FL AIRPORT) / 3 + 3 or 4 Nm to decelerate. At this remaining distance, it’s the top of descente, so I set -3° in “FPA mode”, I also set my target altitude.
Before 10000 ft, I slow down with spoilers not to lose my 3° slope.
When I reach the target altitude I activate approach mode in the MCDU to make the plane decelerate waiting for flaps 1, 2 etc.
To descend, I also manually target the constraints altitudes because the plane actually doesn’t give a damn about.
It’s tricky but It works.
Trick: to find your destination altitude, go to location, never click on the map, search your airport with the keyboard.