WhatsApp and Facebook have been sharing metadata since about 1 year after the acquisition (so, 5 years already). They weren’t public about it, but they have acknowledged it.

I don’t know if that applies to Europe as well; they promised European regulators they wouldn’t, but (a) it was before GDPR, and (b) it’s easier to get forgiveness than permission.

If I had to guess, I’d say they have shared it for Europeans before GDPR and perhaps stopped since. Perhaps not. And perhaps they have this weird bug that copies WhatsApp metadata to Facebook, like many other accidental bugs that help their cause.