Thanks Daniel. I tested this out a bit more thoroughly and dragging from Nuendo into a new project with one track and one empty flow worked perfectly every time.
From the same Nuendo project into my original, quite busy, Dorico project (24 flows, 5 players) I couldn’t get anything to drag into an existing flow (even if that particular track was empty in that flow). Every time the busy wheel would come up, and that action appeared in the undo menu - but I couldn’t see anything actually appearing in that flow.
I tried opening an empty flow at the end, and I could drag into some tracks, and not others. I also got a few crashes when dragging the same thing into different tracks.
I realise that’s not massively helpful in diagnosing what’s going on…is there anything I could do to help you narrow it down?