After the end of the military rule in Brazil, several secret files were made public, including Brazilian intelligence reports on the war. The Brazilian military was completely dumbfounded by the actions of their Argentine counterparts. Early on, the Brazilian military intelligence concluded that if Argentina played their cards right, they had decent chances at keeping the islands, as the British would have only one shot to take it back.
But even if they Argentine held the islands, the main issue is that the Brazilian military simply could not see what the actual desired end game was. So Argentina kept the Island, OK, now what? Do they become a Soviet client state after that and a complete outcast in the South Cone? Because diplomatically and economically speaking, ostracism was almost certain after a victory.
The whole thing made absolutely no sense. It was poorly thought out, poorly executed and had no long term goals. It was all done "in the heat of the moment".