Loaf shrinking after initial oven spring
I have been making Jeffery Hamelmann’s Vermont Sourdough bread for a few weeks now. I have noticed that it has great oven spring when i check it about 12 minutes into baking, just as it begins to start to color, but by the time I have taken it out of the oven it seems smaller than the first time I checked it. What could be the cause of that?
Here is the formula:
1lb, 12.8 Oz bread flour (90%)
3.2 Oz rye flour (10%)
1lb, 4.8oz Water (65%)
.6oz salt (1.9%)
There is a 12 hour liquid Levain build, then mix it all except the salt. 30-60 minute autolyse, then add salt. 2.5 hour bulk fermentation with 1 or 2 folds depending on the dough strength, as well as I can tell. Shape into two boules, final fermentation for about 2.5 hours. Bake at 440 (original recipe says 460, but it turned out way darker than I am used to. I want to get an oven thermometer, but for now I am setting my oven at 440), steam with cast iron and boiling water for first 12 minutes. Bake for 35 minutes (original says 40 to 45, but again, way darker than I am used to).
I am not sure if my scoring is very good, either, I also want to get a real lame.
Any suggestions would be great, and I can share more info if needed!