Responses
Well, this is still ongoing so let’s hope it will be explained in next chapters.
But I think it depend on his mind/heart. Something like the more you are scared and unsettled, the more sickness you will get.
But when Yuu totally made him feels like he belong there with Yuu, Clyde’s mind/heart have an anchor to keep him stay there.

But I wish there's some sort of explanation when he almost disappeared. Because it just went like "oop say bye bye now hehe SIKKEEEE just kidding. Now fuck."