For glissandi I use headless notes to indicate the beat. To chop off them heads I use the transparency parameter in the color/properties panel.
In the full score in both Write and Engrave mode the noteheads disappear. When I change to a part, they reappear again.
Is this correct behaviour?
You need to Propagate Properties to other layouts. Select the noteheads you’ve altered, Edit—Propagate
You need to select the notes in the score and go Edit > Propagate Properties. I don’t know whether the properties you’ve changed are in Write mode or Engrave mode, but you’ll need to be in the relevant mode when you Propagate Properties.
Propagate Properties was introduced as an interim measure, in Dorico 2.1. It will one day be superseded by something more intuitive for keeping overrides aligned across multiple layouts.