Some have suggested that the 2016 PPG reboot would be a good case of corporate fanfiction in the sense that the writers grew up with the original Power Puff Girls, are open about their fannishness and sometimes write their habits into it. Most infamously with Jared Shapiro being based on one of the writers.
When it comes to working with franchises that are often rebooted per generation, sometimes it’s going to be the case. Lauren Faust herself, wife of Craig McCracken, grew up with the My Little Pony toys and cartoons who ended up working on its reboot herself. Perhaps to a given extent as the characters aren’t like G1 (though some suggest they’re closer to their 1990s counterparts).
Rebecca Sugar may have done fan fiction before but have come to do their own works whereas the PPG 2016 reboot would be comparable to superhero fans who get to oversee and manipulate their pet franchises. Like the former, you have the latter bringing in their obsessions and favouritism.
Some even get to sexualise such characters and stuff, which could partly account for PPG16’s woes.