Euwwwwww I'm having the same prob on atty's I've been using for weeks. It's NOT the primer (which I blow out of the atty before using) and doesn't taste like burnt ... so much as burnt filter. M402 and same prob; as soon as it gets the least bit low on juice (with juice still visible under atty bridge, mind you) the taste comes back. It's now 2 different attys, too, on batts and pass-thru.
UGH, it's so bad it makes me wanna vomit. (and NO it's not the analog taste, I still smoke some and they are way better than this, actually smoking last night instead of vaping because of this. This really does make me feel ill.)
Was using carts, then soaked both attys in grain alcohol for an hour or so (long enough to remove the paint from them), blew out, left with tips in VG (batt side down) overnight, put one on this am, dripped, and yep, bad taste comes back within a few vapes. Am now resoaking in grain alcohol, nauseous, and kinda scared to keep using the 3rd atty, lest it happen with this one, too.