Essentially, there are only a few spots on a ghast's hitbox where it can suffocate, and they're in very strange places (the third layer, but not the top layer).
More detailed information can be found at
https://www.youtube.com/watch?v=26T-IcdVmms (1:15 is a good place to start)
Comments 3
It's not actually my video – I just found it and it depicts the problem fairly well.
I did edit my original post though.
Your video is a little long. 🙂 Perhaps you could trim / edit it down to just the part that demonstrates the problem you are having. Lots of tickets to go through. 🙂 If you are unable to edit it, maybe you could just edit your post to indicate what time index we should jump to in the video to begin watching the relevant part. Just a thought.