It started under Martinez, Koeman tried to rectify it by signing Williams and to a point it worked in the first season. Then the introduction of Pickford and Keane have somehow coincidentally seen the inability to defend reappear.
IMO it does come down to training, and not just organisation in the defence for set pieces. It’s positional play and knowing when your team mate needs cover. Yes individual error will always happen but we are open everywhere, teams can just cut through us like butter and these “mistakes” sometime come from being the only option to stop an attack developing into a more dangerous situation.
He’s all about cutting the angles down, rather than a press to the player in possession. Once that’s bypassed, and it has quite easily, its open season on our defence.