Do you know why Japanese porn is censored?
It's strictly to claim that no actual sex is taking place.
If they sold it with a label stating that sex is happening, it would be considered prostitution and therefore illegal.
That's why they censor it—so that, on the surface, it appears no sex is happening. They sell it as if it's just simulated acts.
The most likely explanation for this Fantia issue is that an antisocial organization started selling sex videos to raise funds.
They were posting uncensored videos until the platform administrators issued correction notices every time a new video was uploaded.
The police had been keeping an eye on them, and it seems the illustration community got caught in the crossfire.