The flat earth answer seems to be "perspective"
https://www.theflatearthsociety.org/tiki/tiki-index.php?page=The+Setting+of+the+Sun"A flock of birds, when passing over a flat or marshy country, always appears to descend is it recedes; and if the flock is extensive, the first bird appears lower or nearer to the horizon than the last, although they are at the same actual altitude above the earth immediately beneath them."
This is actually true of course, objects in the sky do indeed appear to get closer to the horizon as they move away from us. BUT they also get smaller because that's what perspective does. The further an object is away from us, the smaller it gets. But that is not what we see with a sunset, we don't see the sun getting smaller and smaller till it disappears, it just sinks slowly below the horizon. Because the sun isn't getting further away, the globe is rotating.
The article I linked to above goes on to say:
"This finite distance to the vanishing point is what allows ships to ascend into horizon and disappear as their hulls intersect with the vanishing point"
That is just plain wrong. That is not how perspective works. Perspective just makes things smaller. The further something gets away the smaller and less clear it will be but so long as there is clear line of sight all the object will still be visible and optical zoom will make it clearer. If the above were true and the reason for sunsets then zooming in to sunsets should show the whole of the sun but it doesn't.
I have seen photos posted on here with clouds clearly lit from below which clearly demonstrates that the sun really is below the level of the clouds.
This one for example where the mountain is casting a shadow upwards and either side the clouds are illuminated:
I have yet to see a flat earth explanation of that.
I've also seen video (annoyingly I couldn't find it on YouTube, it was in a recent BBC series) where they took a plane capable of moving faster than the earth spins, travelled towards sunset, went fast enough so the sun was still relative to the horizon (because they were going as fast as the earth spins) and then went faster so the sun appeared to rise because they were catching up with it. Again, I don't believe any flat earth model could explain how that worked.