I've brought it up before, but I think the well-known medical/training issues at Juventus have been a massive issue through Douglas Luiz's whole ordeal, and explain a lot. Luiz himself implied there was something strange about his injuries and that there's an explanation he didn't want to mention.
In a recent Gianni Balzarini video, he was answering fan questions about why Juventus can't rehabilitate their relationship with different players and develop than rather sell them. Balzarini seems to be a bit of an insider, and so he comes off as biased toward the team's verion of things. He concedes that Luiz joined the club with a minor issue that needed attention, and the staff probably made a mistake in ignoring it and making him train normally, but he doesn't think that could possibly be the whole issue. no, no. There must be some something else we don't know about, maybe in Douglas Luiz's character, and his failure to turn up to training could be evidence of that.
I suspect there are NDAs involved. Plenty of players have had repeated muscle and fitness issues, and often dance around the issue, like praising their national team's medical staff, dot dot dot. Other people have to raise concerns for them, or people on loan elsewhere are observed to have fitness issues. I think Luiz probably confronted the club about their mismanagement and training, and they don't take kindly to that.
If you look into it, Juventus has a weird emphasis on intense strength training and often does double gym sessions. Their players are physically and mentally fatigued, and they're churning through good players who are being labeled busts. For some reason the culture at Juventus is locked into a training philosophy that's hurting their players and they then blame the players for not being tough enough. I'd want to get out of there too.