the portuguese had very similiar results in aoestats pre de which had huge amount of sample data so the change of an statistical error in these findings seems to be low, since hte findings in this new data just from april coresponds to the prevoius findings.
many civs have received buffs, especially all other bad win rate civs in aoestats data, this allone gives a plausivle explanation while the portugese tanked even a bit further in their winrate (+feitoria buff is mostly useless and organ guns are bugged in this patch)
But you wanted some points why the civs is the worst so i share my opinion.
portuguese have a total of 1 bonus on land maps 1v1 and 15% gold discount is much better on pper than in an actual game, since techs are a major part of spending and gold doesnt matter much until castle age. Also this 15% bonus just allows you to train portuguese all around medicre units, which just doesnt cut it. THe bonus is too unfocused to be of any real use.
Examples:
if berbes go cav, they have a much better bonus,
if mayans go archer, they have a much better bonus
their UU is currently bugged since the lat patch further adding to their bad performance but generallz organ guns just dont perform very well outside of early castle snowball (strong and no upgrades but just too expensive later on and esaily countered by mass trash hussar, mass trash skirms or some siege rams or onagers.)
the civ is on the slow side like teutons and their cav is medioce, their archers are average and their infatrz lacks the speed tech.
yay they have good hand cannons but how many time are these units even used, they are niche, same for bombard cannons, good to snipe enemy siege, but they are also the most expensive unit in the game, and if you go mass gunpowder and lose them, which can happen really fast against onagers or cav, or even archers, it is devastating.
Feitoria is more or less useless.
Ah and lastly on water maps we have a caravel unit that is in any 1v1 setting USELESS since galleons are just better. 60v60 ship battles do not happen and even then the carvel is not even broken as has been show a byz FU navy of 40 galleon 10 fast fire 10 demos beats any portuguese 60 pop FU navy compostion.