Not that I'm aware of, and I follow that topic pretty closely. The Signal protocol is the gold standard of messaging encryption, and the whole point is that the encrypted content can only be viewed by the conversation participants. Aside from Signal, the Signal protocol (double-ratchet) is used in many other secure messengers, including SimpleX, and the Nostr-based Keychat. It's fully open source, and has gone through multiple independent security audits. If there was a backdoor, it would have been exposed by now given all the independent eyes that have poured over the code, and the seperate cryptographic audits it has gone through.