After subscribing to the collections/update webhook, Shopify sends the webhook when I update any metafield that is related to the collection. But the metafields property does not exist in the request body.(Rest Admin API)
I'd checked with the product team on this and it does seem that unlike the products resource, collection metafields are not accessible via a webhooks payload. Instead you could make a separate API call to the Admin API to access a collection's metafield.
I do understand that this inconsistency is unexpected though, so I'll connect with the relevant product team to include this as a feature request and we'll update the docs to describe this current behaviour.