https://www.mediawiki.org/wiki/Talk:Design was using LiquidThreads. Adding this page to $wgFlowOccupyPages did not work, apparently you still saw the LQT version of the page, although action=history would show the Flow-style history. Presumably LQT's handling of page actions "won" over Flow.
Setting this to low priority for now since the way we Flow-enable pages will change.