Pick any version of the story. Or read their blog post:
https://blog.whatsapp.com/Keeping-WhatsApp-Personal-and-Priv...
How do they know a message is forwarded? The encryption is meant to make identical plaintexts encrypt to different ciphertexts, so obviously they must be leaking the forwarding status in unencrypted parts of the message. And why is an encrypted service trying to combat misinformation to start with - isn't that a contradiction in terms? These things raise difficult questions. You'd hope that once a service decides to go fully encrypted, its staff would believe that what kind of information going over it or how accurate that is, isn't any longer their concern.
Today, Signal is claiming their encryption means the only data they have to give to government is date of install and last use. In the past they also claimed WhatsApp uses the same cryptography as them, at least for messages. These two claims cannot both be true. If there's some incredibly subtle detail that means deliberately exposing forwarding metadata in WhatsApp but not Signal they should really clarify that because it's not something I've ever seen a discussion of, and it doesn't follow from the cryptography they're using.