[Corazoor]

As far as I can tell: yes. The top z-layer of designated stairs will always become pure down, the bottom layer pure up.

But I think you can get around that by designating one more z-level than you need, and then removing that. Blueprint mode or designation priorities might also work.

Tell you what, I am going to try now, will post my results shortly.

Edit: Yes this works. After finishing the designation, it will change to the corresponding stair type. By doing one more z-level, you move the "down/up only" layer there, and can remove it. The designations in the level below will not change as a result.

For anyone wondering: Yes, this is still an improvement over the old way of doing it.