I dunno... You'd be hard pressed to find an interview with Cavill on The Witcher where he doesn't talk about respecting the source material. He clearly cared a lot about it, because the fans do, and he's a fan himself. The writers/showrunners took a different route and completely disrespected the source material, which unsurprisingly turned it to shit. So makes sense he'd be a bit upset about it, It was a passion project for him.
I misspoke; the issue for me is less about 'feuding' and more about 'leaving the show.'
Henry Cavill is the most important person involved in The Witcher. He's the lead actor, and the most famous person in the cast by a long shot. He was a big get when he was originally announced. The lead actor of a show is incredibly important in Hollywood. Shows (and movies) are not in the habit of recasting. When an actor wants to leave a major show, 95% of the time the role is written out. Recasting is just not done, it is very risky and everyone would rather avoid it. The showrunner of The Witcher is Lauren Schidt Hissrich, who is nobody particularly well known. I think this is her first job running a show. She's just someone the producing company hired. The staff writers are a bunch of nobodies.
If Henry Cavill was willing to walk out because he did not like the writing, the logical thing is to get new writers. Writers are expendable. Even the showrunner is expendable; it is quite common to change showrunners, or to have people do short showrunning stints. What isn't common is to have to recast your lead. Because of that, I find it hard to believe this "feud" led to him leaving the show. It makes zero sense to pick your writers over your lead, and it is a risky move for Cavill to quit a show in which he's the lead, regardless of the reason. Let alone for one like this one, which nobody in Hollywood is going to respect. That's why I think it's being blown out of proportion.