The attachment is a page of a viola part, with cues displayed on an ossia staff.
(1) The first cue on the page, bb34–35, is not a Dorico cue: the material is split between two staves of the piano part and I found it impossible to make it display correctly as a cue, so re-entered the material as an ossia. No matter — it works. The remaining cues are genuine Dorico cues.
(2) What doesn’t work despite a lot of trial and (mostly) error is the duplicated clefs ahead of b39, in b40 and b43 (beginning of system), and in b50. In bb 39 and 50 I can select and delete the clef on the right. This causes both clefs to disappear. Ctrl-Z restores two clefs in b39 and one clef in b50; but presently b50 displays two clefs again. — I realise I may be pushing the boundaries of Dorico’s cue function, but there does seem to be a bug or bugs here.
Another, unrelated problem: my first attempt to produce a png graphic of this page resulted in a completely black page. When I selected mono instead of color, the graphic was created correctly.
And yet another: a lot of the time I find it impossible to select cue notes in Write or Engrave mode, e.g. to hide unnecessary accidentals. At other times it works. Again I suspect a bug, though an intermittent one.
@Daniel: I can mail you the Dorico file if it would be helpful.
I will definitely need to see the project file to diagnose the problem with the duplicate clefs.
In general you can’t select notes inside cues: you can select them in Engrave mode for the sole purpose of respelling them enharmonically, but it’s not possible to specify that they should hide or show an accidental, and you cannot ever select them in Write mode.
Thanks Daniel. I’ve emailed you the project file.
Re selecting notes in cues: I am definitely able to select notes in cues at times, and right now, in the file I just sent you, I can select cue notes in Engrave mode and not only change their enharmonic spelling, but also hide/show accidentals and even adjust the appearance of tuplets. Perhaps it is this intermittent ability that is the intermittent bug 
The problem was caused by an unwanted interaction between explicit clefs entered in the ossia staff and clefs automatically (but inconsistently?) displayed by the cues themselves. It was solved by explicitly setting the required clef of each cue in the ossia staff and then setting Transposed clef > None in each cue’s Properties. This gets rid of not only the duplicated clefs but also the unwanted (by me anyway) clefs followed by overrides at the beginning of systems where cues cross system breaks. Thanks Daniel for help via email!
I can still select cue notes and other objects in Engrave mode and do things with them that the Operation Manual doesn’t think I should be able to do (see post above).
You can select cued notes individually in Engrave mode, by design, as stated above, so that you can respell them. Other operations you might perform via the Properties panel could have unintended side-effects, so I would advise caution in going beyond respelling.
