Translation:Several soldiers fainted in the sun.
The only time I would use "under the sun" in this kind of sentence is if there were an adjective included, e.g. "under the hot sun" "under the midday sun". Without that descriptor, and as described by others, "under the sun" sort of means "everything". "Several soldiers fainted in the sun" is more natural English.
In English, we are more likely to say "Several soldiers fainted in the sun. The phrase "under the sun" is usually used to refer to everywhere on the surface of the earth. It is the difference between a proper vernacular and a literal translation. Are we to use English or stilted French to answer such exercises?
The issue is noted, notwithstanding the benevolent interpretation of those who want to give you the literal word (under) to make it easy for you to reverse translate. They prefer easy translation from EN>FR rather than realize that the English is broken! So in terms of learning French, it's necessary to raise our awareness that "in the sun" is going to be "sous le soleil" and we may suffer the indignities of those who want to insist that it must be literal (under the sun).
It is an accepted version in the current tree version. Unfortunately, Duolingo is testing multiple versions all shuffled together into this one. Moderators are not allowed access to the other versions so we cannot make a correction there that affects everyone. Your best bet is to post your suggestion using the "My Answer Should Be Accepted" button, not by posting here on the comments page.