NB: In Spanish it's the "Sintra" Convention, in English texts it's always "Cintra" Convention.
In England there was, just as in Portugal according to this text, also a huge outcry about letting French troops go and then able to just turn around and re-enter the war. This, the reaction to Cintra, was one of the reasons that the Baylen convention was not recognized by England; even though Cintra was after Baylen.
On this page, the writer also supplied a link to read the Convention: http://myneighborwellington.blogspot.pt/2013/05/cintra-convention.html
The outrage, the sense of abandonment, the suffered violence felt by the population made it become the judge and executioner at the same time, to the point that still today the Portuguese think in anger and grudge of this part of History and having no regrets.
Much like in Tyrol, the excesses of the French and the popular fight against them still resonates today.