It might just be an odd fluke with the editor itself, especially to present movable blocks, which I've made note of for your account in our CRM. Based on some previous cases I've been able to find when this has occurred for other customers, it has not affected live sendouts after confirming the previews and test sends didn't show the bleeding.
When you do your next live sendout, if you notice the inner background bleeding through at the top and bottom of the emails, please let us know so we can escalate this further with our higher level technical teams.
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