In future you need to look at your MusicXML Import settings before you bring in the notes. My suspicion is that they’ve ALL been brought in with Force Duration turned on. You’ll need to go through them and turn Force Duration off for each one individually in this project.
edit: to clarify, I suspect Dorico’s imported the MusicXML file with default Notation Options, then automatically applied Force Duration to everything. This means that retrospectively fiddling with Notation Options won’t fix things unless you turn off Force Duration on the affected notes.