The key thing here is to understand that there is nothing unethical about resolving ambiguities in software requirements. There are certainly diplomatic ways you can forward your recommendations to your current employer without breaching the confidentiality to your previous employer. If you have signed any Non Disclosure Agreements - NDAs with your previous employer, make sure you do not go against it.
What are required to do here is to make a detailed report of the ambiguities you've discovered without any direct reference to your previous employer's interpretation of the requirements. Make sure your recommendations are genuine and authentic. You may only use your previous employer's interpretation as a guide.
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