Why did this happen
So I've taken a step back from sourdough and baking in general lately so just wanted to get some advice as to why this might have happened.
The dough is 75% hydration mostly strong white flour with about 50g of wholemeal. Autolysed with slightly warm water as its cold at the moment. Bulk fermentation over 3 hours with several turns each half an hour, the dough really came together and felt very taught, holding its shape nicely during a bench rest (30 mins) with minimal extra flour. Shaped into a boule and left covered in banneton for 6 hours; my timings are a little off as I started somewhat late in the evening. It had easily doubled in size, however, and felt ready to bake.
Turned out onto a hot baking stone and gave it 35mins at 230° with some steam at the start. You can see in other pictures the very top of the bread was starting to burst a little, despite my scoring. Oven spring was ok but not great. Left it to cool all day until I got home from work. Taste is good, not very sour but full of flavour. I would normally retard the final rise in the fridge but it has been cold, as I've mentioned.
I've gone up and down with sourdough quality and know I need to be a little more scientific in measuring key variables etc but what might be the cause of such big variation in crumb? My initial loaves were brilliant, then the crust completely pulled away from the rest of the loaf, and now I'm at a halfway house, where the crust keeps the bread but there are huge pockets just below.