This issue has been in Bedrock for a while, and now that despawning has been fixed and Mobs are much more common, this issue is much more prevalent. So, the issue is that when Ghast are in their shooting animation, but you run out of their range, they "hold onto" the fireball, and they look like they are about to shoot, but they fly around as normal. Basically, when you reenter their range, they fire the ball instantly without their normal scream warning, so you can get shot in the back without even realizing you are in danger. This can be easily replicated by spawning a Ghast in Survival, then quickly switching to Creative, as seen in the screenshot.