Hmm, I think this answer might depend upon how one looks at the matter, because
The Silmarillion is probably more well known as far as the names of these brothers -- and since it's noted in the published Silmarillion that Amras was the youngest son, in a sense one could say it was 'Amras' who was burned as Losgar, because (in the later version of the history) it was the youngest son who was burned.
In other words, if more people think of Amras as the youngest son, which is arguably true given that the 1977
Silmarillion is arguably more widely known and read than
The History of Middle-Earth series, then they might say Amras was burned when describing the later, revised history.
But confusingly, the new story includes a new set of names too: there is no longer anyone named Amras in the new scenario, and both the 6th and 7th children are named (in Quenya)
Ambarussa, which would become
Amros in Sindarin (not Amras, as can be found on the interweb).
But the story goes that Feanor did not like that both sons had the same name Ambarussa, and he called the 7th son
Ambarto rather -- despite that Nerdanel had changed it to
Umbarto. Anyway this son is slain in the burning, and Tolkien notes what he
would have likely been called in Sindarin had he lived (and thus was not called, because he did not live), and that is -- Amrod. However, since he died he was referred to in Sindarin as
Amarthan.Why? because it all depended upon what name was considered rightly given, combined with what name would have prevailed in Middle-earth: Feanor's Ambarto, or Nerdanel's Umbarto; and it was Nerdanel who proved ultimately correct, and Ambarto never got a chance to be rendered as Amrod in Middle-earth. And all the while Ambarussa remains a correct name too, in a sense and despite Feanor's objections, and this would have become Amros in Sindarin. So we end up with:
6th son: Q. Ambarussa S. Amros
7th son: Q. Ambarussa S. Amros -- Ambarussa changed to either Umbarto (S. Amarthan) or Ambarto (S. Amrod) -- and with his death Umbarto/Amarthan was considered rightly given, given its fated meaning. And we arguably should add Telufinwe here as well, as that meaning goes with the youngest son.
So yes, it was Amrod, the youngest, who was burned at Losgar -- even though he was never called that (dying before he spoke Sindarin in any case), and there is no one named Amras -- but to Silmarillion readers the 7th and youngest son is named Amras!
Delightfully confusing