You're upgrading a SHOULD in the current spec to a MUST.  Has this been discussed on-list?

Speaking as the http.sys owner for Windows, this concerns me.  We don't know the content of the entity body fragments or response headers an application hands us, only the order.  The only way we can definitively comply with this MUST is to make all PUSH_PROMISE frames precede all DATA or HEADERS frames.  As a SHOULD, we're free to leave proper behavior to the app using our APIs, while maintaining non-optional protocol compliance at our layer.