That is so strange! Maybe get ground flax, or leave it out of their diet for a little bit, just to see what happens. If he was bringing up other seed, I'd say he was sick, but this? I've never heard of them bringing up only 1 kind of seed. And he's tapping it out... not throwing up.
Yeah, I'd keep it out of the diet for a bit. If they need the flax, order ground flax, it's easier to digest. Then keep an eye on him to make sure he's not doing that with other kinds of seed.