Q. I wonder if the adverb "at first" in (a) is used naturally being in harmony with the use of past perfect? To me, (b) seems correct, but (a) feel awkward. This is because while the past perfect sentence implies that he had had wrong belief for a long time for that island, "at first" indicates the time when he heard the news.
(a) The floating garbage patch was discovered in 1997 by a man. At first, he had mistaken it for an island.
(b) I hadn't seen him in years. At first, I didn't recognize him because he had lost a great deal of weight.