I am also having this problem on Windows 10. This is extremely frustrating. I created an arena to trap a ghast from moving and have directly hit the ghast countless times with it's own fireball and it has not died.
I am also having this problem on Windows 10. This is extremely frustrating. I created an arena to trap a ghast from moving and have directly hit the ghast countless times with it's own fireball and it has not died.