AFAIK the Cintra Event consists of Vimeiro battle +
So the battle has happened though you have not seen it, and you cannot control it, not even avoid it, as Junot's Armee is fixed. This army is doomed, the game is designed in this way as this is what happened historically. What it is wrong is that it is not reported
From what I have read, some events are random, but not all. Some events will happen at a given time always, such us this or the arrival of Napoleon with the Grande Armee.
But do not worry, your army will come back! If you read what happened at Cintra Convention in the wikipedia link, after the defeat of Vimeiro the french were transported safely to France by british vessels!
But perhaps PhilThib could clarify? In fact it is annoying, it vanishes without any explanation