I have run across a problem. If I have a phrase with a triplet in it, and copy the lyrics using “duplicate to staff below” the lyrics shift out of their correct position.
It works fine if I just copy and paste the lyrics; I wonder why it does not do the same with the “duplicate to staff”. Example attached.
Did you make sure that the triplet was selected before copying?
If you select the triplet (number and/or bracket) along with the lyrics, their rhythmic value is copied correctly.
I also would like Dorico to do this automatically, it’s a bit annoying. Especially since you have to re-select the tuplet again if you are duplicating the lyrics to several staves below. Currently, the lyrics remain selected in the duplicate, but the tuplet doesn’t.
I have a question to the developers, on a more fundamental level: why do tuplets behave like independent objects (like they also did in Sibelius, to my annoyance TBH)? Why aren’t tuplets merely a notational device for representation of non-binary subdivisions? IMHO, say, a group of 3 quavers in a triplet does not actually consist of quavers (eighths), but of twelfth notes, for which the tuplet notation is just a way of writing them in our established binary system.
If I select and copy a passage only consisting of binary rhythms (say, a quarter/crotchet followed by two eighths/quavers), I can paste that rhythm anywhere, and the durations will be preserved, even if the notation gets considerably different from the original. Dorico is good in doing the math.
But if I select and copy a 1/12 note, I actually would like to be able to paste that same twelfth note elsewhere, and let Dorico again do the math of determining the best notation, which in some cases will involve adding a triplet around it, or in some other cases just fitting my twelfth note inside an existing triplet. I mean: the tuplet should not be a thing in itself, ‘containing’ notes, but the result of notating a non-binary subdivision.
(I see Vaughan already gave a useful answer).
We did of course consider making it possible for notes to have irrational durations without need for tuplets, but the problem with that is that you can quickly end up with durations for notes and rests that are non-notatable. If you were able to create a 1/12th note, that would massively complicate all of Dorico’s automatic notation features to produce correct metrical groupings, the ability to use Insert mode, and so on. Having a complete tuplet enforces that there will be a rational duration so that everything can be notated sensibly.
Thanks, Daniel, for your explanation. I understand it would be a huge challenge to implement. It would be useful, however, if a selection of notes (or lyrics) in a tuplet would always include the tuplet, as it defines the actual durations of those notes/lyrics, just like a tie, or a dot behind a note. I can hardly think of a situation where I would want to copy a 1/12 note duration and paste it elsewhere as 1/8.
Allow me one nerdy remark 
: 1/12 is not an irrational number. It’s still an ordinary fraction, just not binary. Numbers like √2, π, or the Golden Ratio are irrational.
OK, thanks all for the answer. I did not select the triplet when duplicating to the staff below. But then, I did not expect to have to. This does seem to me to be both unexpected behaviour and not at all intuitive.
There must be some difference between “duplicate to the staff below” and simply copying the lyrics and pasting them, which does work without selecting the triplet. I wonder why the “paste special” function acts differently to “paste”.