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That’s unfortunate if true, since there’s no evidence isoflavones have any significant cross reactivity in humans.
However I’m a doctor and haven’t ever seen this advice, so I’m skeptical it’s “not uncommon”.
Admittedly anecdotal, but every woman I personally know as a breast cancer survivor has been told this. Some went to MD Anderson even. I do share research articles with them to show them otherwise. Some are receptive, some are not.