The [View history] tab for a Flow board shows the board history, as it should.
But if you want to see the history of the wikitext of the page before it was a Flow board, or if you want to see the "/* Taken over by Flow */" edit in which it became a Flow board, you can't. Flow usurps action=history to mean show board history.
I don't see any good reason to do this. LQT also reuses the [View history] tab, but it changes it to invoke ?lqt_method=talkpage_history; Flow should similarly invoke its own action (e.g. action=board-history or action=flow_history ?), so that you can still query action=history.
Note that we think we will normally move existing content to /Archive(N+1) subpage when Flowifying or converting from LQT, so you can get the page history before it was Flow at the archived page. So this isn't critical, but again there's no good reason to cut off action=history.
Workaround: use Special:Export and uncheck "Include only the current revision, not the full history." Note that using the API (action=query&prop=revisions &rvprop=contentmodel|comment ) to get revision history gives wrong information for revisions prior to a Flow board, bug 73537.