I looked into this.
Fade from niente works in Dorico 4 but not in Dorico 5 because it uses a “zero velocity” for the niente note-on.
Since zero velocity is a note-off by the MIDI standard, the note-ons are registered as note-offs (note-on velocity must be clamped between 1 and 127).
Consequently, fading out works, but not fading in.