I'm prefacing this with that I have only read other comments about possible reasons he would step down and I have no first hand information about this.
In many interviews they have alluded to him being very knowledgeable about the source material and possibly even correcting things when the writers get things wrong. He's very passionate about getting things right.
https://www.dualshockers.com/henry-...ble-on-set-says-freya-allan-in-new-interview/
There are also comments that the writers for The Witcher don't really like or care about the source material.
https://gamerant.com/the-witcher-writer-recalls-staff-mocking-source-material/
Kotaku has an article about the fan speculations as to why Henry left. Simply that he does not agree with the direction they are taking the show and that it deviates from the source material.
Witcher Fans Are Convinced They Know the Real Reason Henry Cavill Left The Netflix Series
So if anything, him leaving would be the most true to the fandom that he could do. The fandom don't want an "Inspired by" series, they want The Witcher. If he stayed and just went through the motion while him and the fans felt that the story was ruined, that would feel more like a betrayal.
How much can you deviate from source material before the fans start to actively dislike the content? So him leaving now is with his head held high and saying that he tried to give the fans what they wanted but that now it is down to wherever the writers and the studio wants to take it.
Do they want it to end like Game of Thrones (a show that so many spoke about all the time but that no one even remembers anymore) or do they want to try to keep it going as best they can?