If I have a series of triplet quavers, the first three groups with 3 above them, and the rest with only 3:2 signposts, and Alt copy them to a stave below, only the first three groups are copied as triplets, the rest are copied as duplets.
Is this behaviour intentional?
If you didn’t select the tuplet signposts, then yes: Dorico copies what’s selected, and if the tuplet “container” isn’t selected, the originally-tuplet notes become normal notes.
As you select notes, if the signposts are showing, they get selected too; otherwise not. If you’ve selected all the notes but forgot to show signposts, you can show them and then Shift-click on a note to add all the signposts to the selection.