I have 3 2019 W Proof Cents with what are very obvious struck through debris. I have looked at them under higher power and they are not scratches on the coin or the die. Two have markings on the obverse and one on the reverse. The third coin also has a strike through on the shoulder of Lincoln.
Mr. P. I've never given much thought to fibers struck through a proof coin. I wonder if they may be from the gloves that the Mint employees wear when processing them.