I think this should not happen → attached page
Indeed, ideally this shouldn’t happen. The reason that it can happen is a little technical, and due to the fact that Dorico has to cast off the music using an estimate of how much music it can fit vertically onto a page, and it may then discover as it does all of the fine positioning of items after casting off that things take up more space than it initially thought, with the result that the staves can end up positioned more closely together than they should be.
You may find that reducing one or two of the values on the Vertical Spacing page of Layout Options, even by only half a space or so, produces a better result. And in 1.0.10 you will be able to adjust the staff spacing manually as well, before the end of the month.