Metadata (who you speak to, how often, etc.) are NOT encrypted (and subject of personalization, marketing, improving app performance etc.).

https://ssd.eff.org/module/how-to-use-whatsapp

P.S. if shocked with this, also check your Google activity history

https://myactivity.google.com/myactivity?pli=1

-- For example, WhatsApp is also end-to-end encrypted, but collects more metadata than Signal.

https://ssd.eff.org/module/attending-protest

Why Metadata Matters https://ssd.eff.org/module/why-metadata-matters

Meta sharing data with Meta? I'm not surprised whatsoever.

Meta not respecting user privacy? Also not surprised.

Are you sure the other person didn’t stalk you on fb?

Yea this happens to me too. The other person probably gave facebook access to their contacts.

That could mean they looked you up on facebook