I call it "plot by stupid" but TV tropes calls it idiot plot. https://tvtropes.org/pmwiki/pmwiki.php/Main/IdiotPlot
Tbh, it's always kinda confused me why hp got such acclaim. They're decent books! I enjoyed reading them several times. However, I wouldn't call them hallmarks of fiction or anything.
Examples? You can't go on a rant and then not give examples.
Hey look, there it is again in the one I’m at now. Hermoine to Potter: “What’s wrong with your hand?” Potter: “Nothing.” (This was the Dolores torture). Hermoine actually finds out, which is refreshing. "You've got to tell Dumbledore." Harry: "No. Dumbledore's got enough on his mind right now." Freaking stupid, Harry.
This “is anything happening?” “No, nothing.” exchange with Potter is constant in this series.
Dude the ball tops of my thigh bones were literally sliding off the bones (they failed to fuse for me) in middle school. I was limping and would have insane attacks if pain when they moved a fraction of a millimeter.
Parents, teachers, friends: what's wrong? Me: nothing
Kids hiding bad things is the most real part.
People hide important things for dumb reasons in the real world, but it's unsatisfying, lazy and overused in fiction. Especially in HP.
Dumbledore thought Harry was a Horcrux mate, he was avoiding him since the start of the year.
Sirius' death is a pretty obvious example
I would like to highlight Harry's character development throughout the series. Although he made several questionable choices in GOF, OOF, and HBP, we should consider that he is a teenager throughout most of the series. Teenagers, umm, well, aren't great decision-makers. As a teenager, I mostly acted mainly on impulse instead of rational thinking.
And talking about individuals defending themselves, not all perspectives find a receptive audience. A perfect example is in OOF, when Dumbledore testifies before the Minister of Magic about Voldemort's return. His statements are met with scepticism and disbelief, mirroring a similar experience Harry had trying to convince others at Hogwarts.
If you want to share counter-examples, I'd be happy to hear them.
Nobody ever explained anything more than the likes of "Voldemort's back! You have to believe me!" If anyone actually gave account or explained anything at all, the good guys would have a lot more allies than they do. Hell, actually talking is how they convinced a bunch of people to let Harry teach them dark arts defense in Order of the Phoenix. It seems that actually talking is how every good event happens, and that not talking is how every bad event happens.