Update: I got around it by reloading to before it happened, and used the console to kill my king while at peace so that France wouldn't be in a war with me. Then I declared a Restoration of Union war on France the same month as they broke off, before the event could fire, and fought to bring them into a personal union a second time.

On the downside, I'm in a regency now from killing the king early, but oh well, I figure a second war and a regency are an acceptable price to pay for keeping France in a union.

This instead leads to the same event firing a few years later when the heir turns 16 and ascends the throne. What the hell is going on? What am I supposed to do to prevent this? Is there no way of keeping France?

Can you provide a save game? I looked over the code and this should not be happening, but MAY (probably) be related to some regency issues we caught the other day caused by the fact that a nation that's lesser in union now lists their overlord's heir as their heir which means for all regency checks the game now thinks they have an heir and executes on flags that may have been set before they were in a Union.

Here's two save files: one where you can kill the king with console to get a regency, and one where you can wait for the heir to age up that year and end the regency. I double-checked by playing from them now, and in both of them I get "France is Free" a month after ruler change.
http://expirebox.com/download/86847c3ed3edbc2a4333065c67783c64.html

Yes, was the regency-related issue I thought it was. This will be fixed with the latest beta release we are putting out later today.

Awesome, thanks!