You won't see any curve at that elevation. The earth is HUGE. That is what I mean by flat earth 'half truths'. You will not see any curvature until you reach 100,000 feet! I explain it in my post here. I hope that is not shameless promotion, but it is on topic.

https://steemit.com/science/@kerriknox/if-the-earth-is-a-sphere-why-does-it-look-so-flat

But yes, there is curvature on the independent balloons, it's just so small that you have to measure it. But part of their 'half truths' is that they just 'eyeball it' and say that it's not curved. But once you have a feel for what the curvature should look like, you can look at the hot air balloons and see the curvature, although it's very slight and really needs a straight edge to measure. I have images of a straight edge taken to those screenshots and the curvature is what would be expected: slight, but there.

Here's a gif I made to show the slight change in curvature from 3000 feet to 100,000 feet. It's really just not that dramatic. And if you think about it, it shouldn't be because 100,000 feet is less than the distance of a marathon. A person in good shape could RUN the distance of those hot air balloons in two hours! It's just not very high if you take the actual incredibly large size of the earth into account.

Good luck. Question everything and confirm with experts on the subject before you believe anything. For instance, there is a post trending on flat earth right now showing that Venus will be visible this weekend and says it's proof they are lying to us, but the article he links to gives a perfectly reasonable explanation for the phenomenon by astronomers. And you can even replicate the phenomenon with a couple of balls and a light source at home to show that it really is possible.

And what is funny is that Venus SHOULD be seen all night every night on a flat earth since there is nothing to obstruct our view and the planets are supposed to be embedded in the firmament. So where does Venus go for the rest of the time when we can't see it?

So, he kind of debunks his own post when you really start thinking about it. :)