I believe it usually happens near the hatch. Humidity that’s too high causes the air cell to be undersized, so they don’t have enough air while they’re pipping
So my ameraucana roo has Had this white thing on his comb for a while now, and I finally looked at it (I’d been putting it off because he’s difficult to hold), and it was a piece of dried corn.
Thereo other bits of corn stuck in different spots on his comb. I have no idea how they got there...
Sex links are good layers, but their egg quality hit bottom at around 2 years due to reproductive problems, so I don't personally recommend them. The breeds I mentioned before are good mothers, breed true, and are also good meat birds