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If you do find_root?? you will see that the final line of code is a Scipy algorithm:

return scipy.optimize.brentq(f, a, b,
                             full_output=full_output, xtol=xtol, rtol=rtol, maxiter=maxiter)

The Scipy doc indicates it should indeed return within the tolerance expected, so I'm not sure what is going on, but at least this is where you should start your search.