I experienced this issue, with two out of four happy ghasts disappearing in a Creative world, but I later found them both unleashed and wandering out of sight. They were initially tethered on a roof to separate fences, and I don’t think they could have suffocated. I think it was version 1.21.94, and it was Bedrock Edition. You should check the areas just out of sight of the fence post for your ghast.
ADELiE Dragon
I experienced this issue, with two out of four happy ghasts disappearing in a Creative world, but I later found them both unleashed and wandering out of sight. They were initially tethered on a roof to separate fences, and I don’t think they could have suffocated. I think it was version 1.21.94, and it was Bedrock Edition. You should check the areas just out of sight of the fence post for your ghast.