Turns out the actual reason my video streaming was broken was an upstream kernel bug where under certain circumstances if a packet too big ICMP was received on one interface but a different interface was sending the traffic, the cache on the egress interface wouldn't be properly updated, so full marks to Fastly for diagnosing that and working to get it upstream: https://lore.kernel.org/netdev/4be64c29-f495-4fdb-a565-2540745d5412@fastly.com/T/
@mjg59 Wait, you had a real-life problem with your TV, and - you managed to debug the problem - the company involved took your debugging info - and they traced it to a bug in an OSS project - and then your problem was indeed fixed by merging a bugfix into that OSS project?
Wow. That is some sci-fi solarpunk utopia happening right there! When will the movie be out?