Middle-loaf problems

Apologies for not having a photo as guidance because the bread was so wonderful delicious that we savaged it immediately.

That said, I've been encountering a problem with some of my boules: good oven spring, big, beautiful open texture when I cut into it, but as I continue to slice towards the middle, the texture gets much closer and tighter. Wondering if anyone else has had this problem.

I follow a basic tartine recipe: 350 g water, 500 g flour, 100 g starter, add 10 g salt with 25 ml water, then stretch and fold, stretch and fold, bench rest, shape, proof for 3-4 hours and bake. Sometimes I overnight the final ferment.