I discovered this while building silentwisperer's nether fortress farm https://www.youtube.com/watch?v=REj2PM6Kw5U and I'm on the step where you put glass panes everywhere. I slabbed 4 chunks out each direction of the farm borders to increase spawn rates, and now I am having ghasts spawn very close to me in the glass-paned area.

I have also observed Ghasts spawning inside of glowstone on the nether ceiling and getting stuck there.