Terror site with no terrorists?

[The Reaver of Darkness]

I just ran an ethereal terror site with only 10 ethereals and no sectopods. I'm on genius difficulty. One of the weirdest things I ever saw!

[Warboy1982]

this is because the terror units are deployed last. if all the spawn nodes are used up by other aliens, there's nowhere for them to spawn so they don't show up. this is especially evident in higher difficulties except superhuman, where i added a clause to force-spawn the extras (because the player is asking for it)

[The Reaver of Darkness]

Does that mean that if large units spawn on a ship that has large spawn points, but they are in the wrong rank slot, they will nevertheless spawn correctly in a place where they fit?

Also, I guess I can edit the map files to increase the number of spawn points. I have very frequently noticed that I get far too few aliens on missions. This terror site, for example, had only ten.

How do I switch on this force-spawn feature, and what exactly does it do?
edit: I have ran tests. Force-spawn puts potentially many units all next to each other at one spawn node.

Thanks for the responses Warboy, very helpful as always!