Polenta Ciabatta

I tried this recipe for sourdough polenta bread with pepitas which was featured here a couple of months ago.

Well, it did not come out as high as the featured loaves.

The loaves just spread out as soon as they were removed from the brotform. Slashing the loaves was like letting the air out of a balloon, so I didn't slash the second batch.

The crumb is nice and full of bubbles, even though it's FLAT. It came out looking like a ciabatta, which I haven't been able to achieve when I try to intentionally make ciabatta!

Anyway, the flatness, how to keep the loaf from flattening out when it is removed from the brotform? This is the biggest problem that I have!