I copied the piano part in the middle and pasted it into the lower piano staff. As you can see, the cross‑staffed notes were not pasted correctly:
Is this a bug, or is there something I should do to make cross‑staff material paste correctly?
It will because, somewhere else, you added then removed an additional staff in your target instrument. Dorico pastes the cross staff notes there (which you will see if you add that staff back again)
(EDIT: If you do make that staff visible again, you can move the notes up the extra staff, so they are now shown on the top staff and then remove the extra staff again!)
Hi @prko, I have seen different results when copy pasting cross staff music. It depends if your original music was all in the lower or in the upper staff. Try to select another staff (lower or upper) when pasting, and see if the result is better.
Otherwise please upload an example, to experiment.
Edit: But @Janus may be on a better way with his suggestion above.