When you hit reply in an email client, that email client is what actually chooses what pieces of code are over-written, and which are left alone. There's no consistent methodology, so making the outcome consistent is nearly impossible. If you try sending yourself this test in several different email clients, and respond from all of them, I'm fairly certain that you'll see different changes from all of them, and some may very well have different containers to keep background colors from bleeding over like you're seeing.
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