Q.What Causes The Brown Core In My Liberty Apples?
It isn’t in every apple, but more than half. They look fine from the outside, but when I cut into them it looks like a bruise with brown discoloration right around the core. Brown flesh is firm, not soft like a bruise. I live in north Idaho. I pick the apples the last week of September.

Certified GKH Gardening Expert
This appears to be related to, either, maggots or caterpillars! Here are some articles that will help:
https://www.gardeningknowhow.com/edible/fruits/apples/apple-maggots.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/prevent-caterpillars.htm