When simmering a basic chicken or beef broth (bones/meat/onion/carrot/spices) the result generally improves with time until, often, after a long time (on the order of 12-24 hours) the broth and solids change color significantly in the 'dark' direction and it smells a bit, well, burnt. Not 'gross throw it out', but definitely a 'darker' aroma and appearance.
I'm confused about how this happens. There's still significant liquid in the pot so why is anything burning? The temperature should still be hanging out close to 100C/212F. If color wasn't developing before, why does it eventually develop now? Does it have to do with any solid pieces poking up above the waterline? I wouldn't expect those to have such a different temperature.
Any tips to prevent this? Or is this normal/expected/desired for long 'bone broth' type soups? (Or does this only happen to me?)