Mastodon requires the updated timestamp field to be present on updated statuses, to prevent conflicts and race conditions where two “Update” payloads get sent out and the receiving server doesn’t know which one to use. You can see this requirement here: mastodon/process_status_update_service.rb at d66dfc7b3c1b62a0d5276387ea8745da598afacc · mastodon/mastodon · GitHub
I’m not sure if this is documented anywhere, but I’ll look into it and see if I can update the documentation accordingly