It's a shame this kind of thing is so hard to prove, otherwise it would be all over the media. People will write it off as 'coincidence'. "Perhaps you looked for or discussed it elsewhere".

What happened with Skype before was that Microsoft would ping any links from their servers, so it was really easy to prove it by generating a new web server, publishing it nowhere and then mentioning it in a chat. This caused some publicity and they stopped the practice. Skype didn't guarantee E2EE at that time though.

But perhaps you could do a similar 'clean room' excerise to prove it. I don't think they would break the E2E by the way but perhaps there is something calling home in the app itself.