I don't work for either Netflix or Apple, but this is the most reasonable explanation. Some things that I do know:
I have verified that I can still use an IOS 10.x Ipod to stream Netflix downloads to two different TVs using the Apple lightning-to-HDMI adapter. I cannot use these same adapters to watch these same movies in Netflix on any of my 11.x iPods or iPhones, and my family has several.
Second: the symptom, running in Netflix and streaming audio but not video is 100% compatible with HDCP-incompatibility problems that I have seen with a variety of devices going back at least 5 years. Without wanting to explain what HDCP is and how it works, it was designed to protect digital content, by _trying_ to ensure that a pirate can't put a recording device between the source (your iPhone) and the TV.
My guess that the weak link in this chain happens to be the Lightning-to-HDMI converter, which actually contains a small ARM computer and it quite likely NOT to be considered safe enough to protect digital content. That said, it is also certain that what has changed in my configuration is the IOS software, not the adapters. Without having been able to find any place where Apple admits to having "fixed" the IOS driver that talks to that adapter, that is far and away the most likely cause that this PhD computer scientist can identify.
I would not wait for this to get "fixed." If anything, the digital content protection people consider the IOS 11.x solution a fix of the previous hole that may have - and probably did - allow people to illegally copy protected content. I doubt that Apple is going to rush to find a way to get an HDCP-compliant adapter to market, both because it will be very hard for them to hit the already-getting-expensive price point of the current adapter and because none of the major competitor phones provide any similar ability to stream content over a wire, without relying upon some sort of existing networking infrastructure (with the POSSIBLE exception of SAMSUNG phones being able to talk to SAMSUNG TVs. On my Galaxy 6S, though, this seems to be a lot like Chromecast, in the sense that the phone doesn't send the content to the TV, it just brokers a connection between a wireless router and the TV.)
I hope I'm wrong.