That is strange… According to the documentation that you linked to, this only applies if “your app accepts h2c requests”, which is atypical.
Possibly the proxy code has drifted out of synch with the docs site, but it might help to post your full fly.toml…
Also, if your users could try the flyio-debug trick, that might help narrow down the source of the differences (even if it doesn’t solve it all the way).
Hope this helps a little!