I wrote canh sắt and was not alerted that anything was wrong. This is happening A LOT. It would really be a major improvement if wrong accents were always pointed out (like happens for some sentences, especially earlier in the course).
A doublet of là (“to be”); see there for more. This is an innovation in Vietnamese, not found in Mường and other Vietic languages. For the semantic relation, compare Chinese 為 (vi, “to do; to be”).