I previously asked a question about the White Bread with Poolish from Ken Forkish's book Flour Water Salt Yeast. My first attempt with a flour that had 10% protein resulted in dough that hold its shape, but the poolish didn't expand enough so flavor wasn't as great. So, I decided to use flour from my bakery and asked for baguette flour. I don't know anything about its protein content, but I imagine its bread flour. When using this flour, my dough gets good elasticity when I am folding it and a poolish that nearly triples in size, but after the bulk fermentation it's no longer as elastic which makes it impossible to shape. I know bread flour absorbs more water so if that is the case then shouldn't it be firmer and more elastic. I measured everything by weight. Should I go for a poolish with bread flour and try the rest with all purpose? But, it doesn't make sense why would it be less elastic. I imagine the only explanation would be that the "bread flour" has less than 10% of protein. Any thoughts? The hydration level of the bread is 75%. The bread tastes good in the end, it's just a hard crust. Anyone else done this recipe?
The protein content of your flour may have something to do with it, but it may also be the fermentation of your poolish. The more fermentation a dough goes through, the more the yeast turn starches into acid. Acids can denature gluten and gluten-forming proteins, leading to a less elastic dough. When you're adding your poolish to the rest of the ingredients, you're also adding its acid. The poolish is always going to be goopy and loose, so switching to bread flour for the poolish may not have the effect you desire.
Unfortunately, I'm more or less in the same boat. Like you mentioned, a higher protein content may help. That said (if I remember correctly), Forkish recommends AP flour for all of his recipes. Perhaps a shorter poolish ferment, or shorter bulk ferment.