Okay, here’s my 2 cents:
1) Top critics didn’t outright pan the film, and I think the harshness of the criticism for Crimes of Grindelwald is inconsistent with the leniency the original franchise was met with - if pacing and editing as it affects the plotting were the problem, why weren’t those criticisms made to the same extent against the original franchise?
On top of that, it’s widely acknowledged among top critics that the directing and design and world-building, thematics, etc, are more accomplished in Crimes of Grindelwald than in many of the Harry Potter films.
Crimes of Grindelwald was not written off as a bad movie by professional critics, and the over-the-top hatred for the film generally comes from politicized hatred of Rowling and Depp that was whipped up before CoG was released by the likes of The Mary Sue, Scott Weintraub, and other small-time critics, who ended up negatively impacting the RT aggregate score, which depressed the box office from what it could have been, and shaped people’s opinions of the film.
2) There’s a determined infantilization of this franchise by casual fans who think JK Rowling should write stories as “lighthearted” as you put it.
No matter how ridiculous an interpretation of the story and franchise they might have, it is a fact that a certain contingency of people, for egoistic reasons, refuse to credit this series with the maturity it has, and refuses to regard it with the seriousness it so often asks of it’s audience, and thus the plotting often flies right over their heads, leaving them confused and constantly feeling like they were sold on something different by the advertising.
Like, how many times do we have to hear “this one is so dark!!!!” in each new review of a Rowling product. If you went into Fantastic Beasts taking the title literally and wanting Pokemon, I’m sorry, this franchise isn’t for you.