Bottom blowout on baking stone

Hello everyone! 

I used my baking stone for baking bread for the first time today, for a recipe that turned out lovely in my Dutch oven. Unfortunately, these two loaves came out super wonky - the dreaded “bottom blowout!” (The boule is below; the batard is the thumbnail of this post.)

I didn’t know whether to laugh or cry when I took the first loaf out of the oven. XD

After some searching around the forum posts, I think that these two loaves were probably taken out of bulk fermentation too soon, and the shaping wasn’t so good either (especially with the batard). 

Here’s what I did (the recipe is the Simple Sourdough from The Perfect Loaf, though I decided to make a levain):

5-hour levain (in my Sourdough Home at 74 degrees for about 6.5 hours to let it ripen more):

• 53 g starter
• 106 g white flour (50/50 AP/bread flour blend)
• 106 g water (78 F)

 Autolyse (30 min):

• 153 g levain
• 767 g bread flour
• 85 g whole wheat flour
• 537 g water (86 F to reach a desired dough temp of 78 F)

Final mix: 

• 43 g water
• 15 g fine sea salt

Bulk Fermentation (4 hours at 76 degrees in the B&T proofer box):

• I did 3 stretch-and-folds, one every 30 minutes, until the last 2 hours.
• After taking the dough out of the proofer box, I divided and pre shaped, and let it sit on the counter for 35 minutes.
• During the final shaping, I struggled to make clean seams on both loaves. The batard shaping was definitely a bit messier than I would have liked.
• I put the loaves into bannetons and then into the fridge for overnight proofing (approx. 10 hours).

Bake:

• Preheat the oven for 30 minutes to 450 F with the baking stone in the middle of the oven and a roasting pan on the lower rack right above the bottom heating element. (My oven has a top heating element as well, and I can’t control either one.)
• I scored each loaf right out of the fridge. As soon as the first loaf was scored, it went into the oven. The second loaf (the boule) stayed in the fridge until the first one (the batard) was done.
• I sprayed each loaf before sliding them onto the baking stone (on parchment paper). I baked each one individually.
• As soon as the loaf went in, I put 5-6 ice cubes into the roasting pan for steam. The roasting pan was underneath the baking stone.
• I left the steam pan in the oven for the first 15 minutes for each loaf, then took it out.

Possible causes for the “bottom blowout”:

• Bulk fermentation could have gone for 15-30 minutes more.
• Poke test - the dough sprang back a little bit after I took it out of the fridge, but not completely.
• My previous loaves have been overproofed after being in the fridge for up to 19 hours, so I erred on the side of caution and took these two out of the fridge around 10 hours.

I made this same recipe on Friday night but halved to make only one loaf, and it came out beautifully in my Dutch oven. (I did let it bulk ferment for 15 minutes longer until I saw more air bubbles and the dough felt fluffy and jiggled a bit in the bowl.) Here’s the crumb of this loaf:

What do you guys think? Are the two aforementioned loaves underproofed? How do you guys bake directly on a baking stone without this happening?