So how is Shueisha verifying if someone is not using the full-pages from source material, clean panels, gifs, etc, for any of those special cases besides the normal daily use that I mentioned above? The answer is that Shueisha probably doesn't look this deep because they're probably using bots.
What big companies do is claim their copyright whenever able because that's easier and because they're control freaks. Unfortunately, people don't have the means or the time to fight back, ot to go to court, or to do anything at all about it, so the companies just don't bother with the possibility of backlash because they have no incentives to be ethical and lawful.
And given the fact that one of their own partners got hit with a copyright strike (the DBZ one), I'm assuming Shueisha has outsourced their DMCAs to a third-party enforcer that is most likely using a bot to identify art/keywords, hence the speed and randomness.
The 3rd party just throws out takedowns and hope something sticks so they can show how effective they are.
Again: according to the LAW that went into effect January 1st, there's nothing wrong with the use of gifs, minor uses of frames or screenshot.
They're just abusing of their rights as the IP owners to do whatever they want.