I’ve regularly bought Brooks Brothers suits for years, and every single one of them had the exact same Shatnez issue – the collar stiffener, which was 100% linen. Fortunately, it’s a pretty easy fix, but you need to make sure that the one fixing it is Makpid to completely remove the entire old stiffener before sewing the new, non-linen one, in place. It can be easier to just cut the old one out, leaving edges in place, rather than ripping out at the seams.
an Israeli Yid