How/Why Does Broth Burn?
Brown meat broth is a traditional recipe in Brazil, where bones and vegetables are roasted in the oven with some olive oil, before they are used in making the broth. This &quot;caramelizing&quot; effect turns the broth brown.
In your case, and as observed in the comments above, when the brothgets &quot;thicker&quot;, some pieces of vegetable and bones have reacted at the bottom of the pot with the strong direct heat, getting a bit roasted/burned, which gives you the same &quot;caramelizing&quot; effect as in the Brazilian brown broth recipe described above.
hence you describe in your question:

and it smells a bit, well, burnt.

stirring regularly, to make sure no heavier parts of the broth stay in direct contact with the bottom of the pot for too long, avoid the broth &quot;burning&quot;.

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