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I really don't want to speak ill of any team at the moment, though a whole lot of the big teams that were considered to be favourites to win their opening games did flop, the likes of Brazil, Spain, Portugal, Czech, but I would like to give them the benefit of doubt based on the fact that sometimes opening matches could be very hectic but as time goes on the teams start to progress all the way as the competition proceeds from the group stages to the knockout stages.
You are right. The starting game of any team might be tricky just like the opening rounds in each league, you might see top teams dropping points against teams almost relegation in the season before.
There is the famous option of Argentina in the group stage against Saudi Arabia and how they ended winning World Cup in Qatar in 2022.
Nevertheless, this trend of so many upsetting results is weird. I can understand a failure of a team, two, maybe three but here we count the opposite, it is like how many team won while being the favourite one before the match.