Ushuaia is not south enough to experience midnight sun, not even on the day of the southern summer solstice, which means that the sun actually sets. However, it is possible to observe the sun as it sets. It doesn't set directly in the west: according to the flat theory, it sets somewhere in the northwest, and according to the globe theory, it sets somewhere in the southwest. Furthermore, according to the globe theory, there will be no "astronomical night" but a mere twilight. A certain section of the sky remains lighter (or "less dark") even after the sun has set: and that less dark section moves further towards to the south, never really disappearing, and being in the south by midnight; and after midnight, it moves further towards the east, until the sun rises from the southeast. (Or, according to the flat theory, it rises from the northeast.)
There is a very fundamental problem with the entire southern hemisphere in FE theory.
In RE terms, the apparent rotation of just about everything in the sky is the reverse of the direction the Earth rotates in. The sun, moon, stars and planets all rise above the horizon in the East and set again in the West. In the Northern hemisphere - close to the north pole, this means that things like the "midnight sun" describe circles around the sky and they move clockwise around the pole star - which is vertically above your head.
FE theory seems to be able to reproduce those effects by rotating the firmament and moving the sun and moon around appropriately.
But at the south pole, things get decidedly squirly. In RE terms, the celestial bodies still rise in the East and set in the West in the southern hemisphere - but if you're at the south pole, that means that everything is moving in COUNTER-clockwise circles around you at a point above your head.
In FE theory - that simply can't happen. In the version of FE theory shown in the maps on the Wiki, there is no south pole - there is an infinite plane of icy nothingness that surrounds the known world.
In FE versions that DO have a south pole, you should be able to see polaris from there - and the firmament doesn't rotate counter-clockwise around a point above your head.
But what a lot of people are missing is that you don't have to be right at the south pole to see this. (Which is just as well because it's forbidden by massive amounts of UN patrol ships/planes/drones/whatever).
Anyone who's been to Australia or NewZealand will tell you that the night sky looks NOTHING like FE descriptions. You can see the point that everything is rotating around - and polaris isn't it! There is no actual bright star at the south pole of the sky...but if you watch long enough - you can see that the firmament is rotating in a counter-clockwise direction about a point with no star. If you stand reasonably far north in the northern hemisphere - you see the firmament rotating in a clockwise direction about a point with that bright star called "polaris".
It might be tempting for FE'ers to claim an outer ring of the firmament that rotates counter-clockwise - but that doesn't reproduce what you see in the southern latitudes...the rotation would still have it's "center" at the pole star - and it simply doesn't do that.
The ONLY (very half-assed) explanation I've heard is that people in the southern hemisphere are confused and are actually looking North when they think they're looking South!!
Well, I have a cousin who lives in NewZealand and on the couple of trips I went there - I saw with my own eyes how the sky looks - and for 100% sure it doesn't match any of the FE descriptions of it...and yes, I knew full well which way was North.
The fact of the rotation's center being a blank patch of sky - the fact that the direction of motion is counter-clockwise - and the fact that the constellations are completely different there - totally busts the idea of a simple rotation of a fixed firmament.
But I don't expect to hear any good explanations for this...it's a helluva stretch for FE'ers to explain any of these southern-hemisphere phenomena effectively.
(And I still have no credible answer as to how the FE moon produces TWO high tides each day.)