Considering how popular WhatsApp is, it's very hard to believe that there are no security researchers reverse-engineering its crypto code.

Because WhatsApp uses end-to-end encryption, any backdoor must necessarily be on the client side, and all client-side code can ultimately be reverse-engineered. This makes such backdoors very tricky to implement.

With that said, while I think a "general backdoor" (one that weakens the crypto algorithms so much that all messages can ultimately be read by Meta) is super unlikely, a "vulnerability" in some image parsing library, designed and implemented by the NSA, and only used on the most interesting targets... now that's a different story.