I've been making sourdough bread for over a year, with good results, but recently I've had problems with a large air hole at the top of the bread, just below the crust. The only thing I've changed is to move a metal baking tray to a cloche. I'm slitting the bread at the top in the same way, so would expect gases to be able to vent. The cloche is supposed to keep the top of the loaf moist so that it doesn't harden too quickly, so perhaps this isn't working. I've noticed that the cloche base doesn't get as hot as the metal tray did when I preheat it.
I'm inclined to blame the cloche for the problem, but could I be under or over proving the bread? The cloche is certainly not as hard work! Any advice would be welcome.