The site's title is misleading, "leak" makes it sound like microplastics are leaching/flaking into the liquid within an already-sealed bottle.

Instead it seems [0] the contamination originates from exterior bottle-cap paint, where particles fall into the liquid before things are finally sealed. Dusting/rising the caps before the joining causes a measurable improvement.

[0] https://www.anses.fr/en/content/caps-glass-bottles-contamina...