Flatten transparent objects setting introduces a lot of unnecessary artifacts, which severely damages image quality.
Here is an example, at 200% magnification:
FF Core Optimize [Flatten transparent objects: Gradient Resolution: 300 DPI, Downsample color images to 300 DPI]: https://i.imgur.com/p9SfZGt.png
Acrobat [Downsample color images to 300 DPI]: https://i.imgur.com/Bha7qHR.png
This is with "recompress Color/Gray Images" turned off, I suspect with it on the result would have been even worse.