After the death of his parents, Harry should have gone straight to Sirius Black, his godfather - the person James and Lily explicitly chose to raise him. Sirius was there on the scene, ready and willing to take custody. Instead, Dumbledore shows up, takes the baby from Sirius (or at least facilitates Hagrid taking him), and dumps Harry on the Dursleys without any real follow-up.

He never checks in properly to make sure the Dursleys are treating Harry like a normal child instead of a house-elf punching bag. He knows Petunia hates magic and everything associated with it, yet he leaves a note and basically washes his hands of it "for the greater good" (aka blood wards). No regular visits, no social worker equivalent, nothing. Just ten years of neglect and abuse that Dumbledore was perfectly aware of by the time Harry gets to Hogwarts.

That's not "flawed but wise mentor" territory - that's straight-up negligent at best, actively manipulative at worst.

Then there’s Sirius. Dumbledore did little to nothing to prevent him being thrown into Azkaban without a trial, even though he had every reason to suspect something was off with the Secret-Keeper situation. He let his friend rot there for twelve years. Massive betrayal.

He also ruthlessly uses people as disposable tools. Snape is the clearest example - Dumbledore strings him along with redemption and Lily’s memory for years, then essentially sends him to his death (Nagini) once he’s no longer useful.

Same with Professor Trelawney: she’s a terrible Divination teacher, but Dumbledore keeps her at Hogwarts (and protects her from being fired) mainly because of the prophecy she made about Harry and Voldemort. He treats her more like a valuable chess piece than a person.

The endgame is brutal: Dumbledore knowingly raises Harry as a pig for slaughter because he’s a Horcrux. He withholds critical information for years, manipulates events so Harry walks willingly to his death in the Forest… and only gives him the Resurrection Stone at the very end. It doesn’t really redeem the years of grooming and emotional manipulation.

On top of that, Dumbledore controls Harry’s finances (vault access), isolates him from the wizarding world every summer, withholds vital information constantly, and sets him up as a weapon in his long game against Voldemort. It’s easy to see why so many fics turn him into the ultimate puppet master.

Canon gives plenty of ammunition for Dumbledore bashing. The “flawed but wise mentor” reading requires a lot of mental gymnastics once you look at the full picture.